Hey guys, I have been looking around for a dnd book, preferably a sourcebook. I have the three core books, xanathars, tasha's and volos. I am really torn between theros, wildemount, eberron and mordenkain's (tome of foes). Any help guys? 

What are you looking for? I'm sure they're all great for someone who has use for their content (don't own Theros myself, but I know the theme). 

If you don't intend to play in one of the settings you can still mine them for a few mechanics and for their themes, but in that case I'd lean towards Mordenkainen's myself. I like it, it has a good variety of cool monsters and it sheds some light on a few species. 
If you do intend to play in one of the settings, picking up the sourcebook for that setting seems like a no-brainer. 
If you want a book to browse through, regardless of wanting to use the setting wholesale, more than a monster book the setting books all have their qualities. 

You arguably can't really go wrong, but for us to tell you which one will suit you best is next to impossible.  

Mordenkainen's Tome has a ton of lore about the Blood War and several of the major races, along with racial character options and a collection of monsters. The other three are setting books. I love the piety mechanic in Theros and if you want to run a game with a Greek Pantheon feel, that's the one to get. The College of Eloquence and the Oath of Glory were reprinted in Tasha's so if you were looking for PC content there're only a few new races in that one. WIldmount is a continent in the Critical Role setting, which I had no interest in using, but I have taken several city and region entries to use when I needed one. They're pretty good overviews of generic locations that can be placed wherever you want them. The dunamancy spells are pretty interesting too. I absolutely love the Eberron book. Great setting: magipunk, political intrigue, post-war tension, one nation mysteriously nuked off the planet... it just brings me joy. All the 3e Eberron books are still relevant too because they didn't advance the timeline at all. The artificer class was reprinted in Tasha's as well. As pangurjan said, it really depends on what you're looking for.

Hey guys, sorry for not getting back so quick! I would like some new character options and some worldbuilding/lore to go with it. Might not help but if it does then why not

Theros has centaurs, lion people, minotaurs, satyrs, and (reprinted) tritons for race options. There's an athlete background. You already have the two subclasses because they're in Tasha's.
For lore there is a fleshed out pantheon of Greek mythology-inspired gods, 4 city-states and a cosmology, how to run games where the gods directly influence events, some magic items and monsters (including 3 mythic monsters), and a bunch of maps.

WIldmount has pallid elves, (reprinted) sea elves, lotusden halflings, (reprinted) aarakocra, (reprinted) aasimar, draconblood and ravenite dragonborn, (reprinted) firbolgs, (reprinted) genasi, (reprinted) bugbears hobgoblins and goblins, (reprinted) goliaths, (reprinted) kenku, new orcs, (reprinted) tabaxi, and tortles for race options. There's a supernatural gift called the Hollow One. Three subclasses: the echo knight fighter, chronurgist wizard, and graviturgist wizard, along with several new spells and two backgrounds. There's also a system called the heroic chronicle that helps players make characters set in the world. 
For lore there's a bunch of factions and societies, a gazetteer for the different regions of the continent, magic items, monsters, and maps.

Eberron has changlings, kalashtar, shifters, warforged, (reprinted) bugbears hobgoblins and goblins, and orcs as well as 12 dragonmark subraces for race options. You already have the artificer class because of Tasha's. 
As for lore, there's a gazetteer of the continent, several religions, a detailed description of a city, how to run games in the setting along with a short adventure, magic items, monsters, and maps.

Mordenkainen's goes on a deep dive into the Blood War between devils and demons, elves and drow, dwarves and duergar, gith, halflings, and gnomes. There are a bunch of tiefling subraces, demonic boons, eladrin, sea elf, shadar-kai, (reprinted) duergar, githyanki and githzerai, and (reprinted) deep gnomes for race options. There is also a sizable bestiary.

Of the options, Mordenkainen's is the best if you play in the Forgotten Realms setting and use the typical planar cosmology.  The others are written for different settings entirely, and while there are obviously elements that can be considered setting-agnostic in any of them, none of them are really written for the FRs in terms of world-building.

I say hold your money- I  had a gift certificate at GameStop and since they don’t let you buy gift cards with gift cards - couldn’t find anything at all to buy. For whatever reason they had one copy of wildmont in a corner- I bought it-skimmed through it and haven’t touched it since. Unless you really want to play in that setting, don’t bother with setting books (imo).

as far as Mordenkinen’s, only buy it you really want a physical reference for the monsters. The stuff on the bloodwar is available for free https://media.wizards.com/2019/dnd/dragon/27/DRA27_bloodwar.pdf

if you’re talking digital books, just get the monsters-only from mordenkeinen’s-imo.

personally, aside from for those 6 you mentioned, the physical book I’ve opened the most BY FAR is waterdeep dragonheist.  nobility, faction info, a few magic items and monsters, variety of npc’s, a real variety of maps that can be used anywhere-lots of reason to go into that book even if you’re not playing in waterdeep - just imo of course.

There are some parts of theros and wildemount you can pirate, like the piety system in theros, or the in-depth character backstory generation system in wildemount. But in either case, you’d need your DM to want to use the system. So I’d agree that in general, setting books aren’t so great unless you’ll be playing in that system. 
There will be one or two more books this year, iirc. Maybe hold off until they are announced/released and see if they’re more interesting.

The two new books this year have already been announced: the Wild Beyond the Witchlight and Strixhaven: A Curriculum of Chaos (unless you're talking about yet another two which are also expected this year and I just didn't get the news).

Out of the campaign settings I mentioned, which one has the best starter adventure/helper majig? 

Wildemount has four short adventures, one of which is Frozen Sick. I like that one a lot as an intro adventure. Eberron has one, which is a pretty good intro to Sharn.

That said, there's plenty of decent starter adventures to be found for free (Frozen Sick among them) or nearly free - this thread has a few pointers. I wouldn't necessarily base my decision on which setting book has the best of those. 

I'll suggest something totally different and say Midgard Worldbook, Midgard Heroes Handbook, or Tome of Beasts from Kobold press.  If you want an insanely amazing campaign setting, with an interactive online map and an incredible amount of supplementary material, go Worldbook.  If you want character race options (like bearfolk and trollkin,),  subclasses, feats, martial options and spells, go with the MHH (you can use these in any setting).  If you want monsters that will challenge your players with something totally new, and will give you ideas for incredible encounters, go Tome of Beasts (again usable in any setting).  I personally rank all of these books as better than anything you listed.  

Quote from SwiftStormwolf >>

I'll suggest something totally different and say Midgard Worldbook, Midgard Heroes Handbook, or Tome of Beasts from Kobold press.  If you want an insanely amazing campaign setting, with an interactive online map and an incredible amount of supplementary material, go Worldbook.  If you want character race options (like bearfolk and trollkin,),  subclasses, feats, martial options and spells, go with the MHH (you can use these in any setting).  If you want monsters that will challenge your players with something totally new, and will give you ideas for incredible encounters, go Tome of Beasts (again usable in any setting).  I personally rank all of these books as better than anything you listed.  

I assume they want D&D books specifically because they do not want to spend a lot of time using the homebrew tools to integrate the information digitally to this site, especially since they are asking on Beyond rather than Reddit or something.

I would personally got with MTOF since it is the most setting agnostic.

The Midgard Worldbook also runs about $150 nowadays, and that won't go down until (and if) there's more copies printed.

I side with Wildemount, which is one of the best settings (let alone sourcebooks) 5E will ever produce.

• It's well formatted, including spectacular artwork that makes you feel like you're becoming drawn into the setting.
• It has several new spells, backgrounds, subclasses, races, subraces, and even a Heroic Chronicle for the most expertly made backgrounds you will ever receive from an official product.
• It has dozens of new magic items and monsters to gorge on.
• It has 4 - yes, that's right, four new starter adventures for levels 1-3, each one being uniquely themed and containing exotic rules such as grid travel, ships, diseases, and intrigue.
• It has over 100 pages on locations, providing heaps of information and quest hook for each locale featured.
• It has several pages on gods, factions and nations within the setting, and...
• It has a fold-out map at the back that is a fair size larger than the book itself.

Also, it has Sapping Sting, which will irk your enemy no end by halving their movement speed.

Quote from pangurjan >>

The Midgard Worldbook also runs about $150 nowadays, and that won't go down until (and if) there's more copies printed.

 I just checked Kobold Press's store and the hardcover is in stock for $50.

Quote from Buzzardoftheages >>

Quote from pangurjan >>

The Midgard Worldbook also runs about $150 nowadays, and that won't go down until (and if) there's more copies printed.

 I just checked Kobold Press's store and the hardcover is in stock for $50.

I'll take a look, though after shipping and taxes it's probably still going to be twice as much as that for me. 

edit: $125.74. Going to pass on that. At least it's an option if you're not on the wrong side of the ocean.