Each sourcebook adds different features such as races, classes, feats, monsters, and more. However, which 5e sourcebook do you think adds the most unique and fun features? I will be excluding the DMG, MM, and PHB since you basically are required to have those in order to play, I will also not list Legacy content.

I've only got the core rulebooks and Xanathar's Guide, so it would be unfair of I were to vote for it. That being said, XGtE contains a hell of a lot of useful information and systems packed into such a relatively small book. I mostly use it for the random encounter tables, artisan's tools descriptions, and character names, but the subclasses and downtime activities are awesome too.

I plan to buy Tasha's Cauldron at some point, mainly for the Artificer, the additional subclasses, and the sidekick rules. And maybe Monsters of the Multiverse (obviously for the lineages)

Fizban's Treasury of Dragons adds a lot of cool new monster items + lore and descriptions.

I also like Fizban's for the monsters, but I think that MotM overall has much more to offer. Especially since Fizban's is solely based on dragons, and though that is not a bad thing, I think MotM covers more.

Your poll missed the one with the answer.  The Players Handbook.

In my mind the best all around species of choice is Human Variant

And some of the better Feats are in the PHB as well

The majority of these were pretty damn good additions. Explorer's Guide to Wildemount, Fizban's Treasury of Dragons and Van Richten's Guide to Ravenloft were particular well done.

In the end it came down to Xanathar's and Tasha's. Tasha's has a lot going for it (it nearly got my vote just for fixing the Ranger!) but in the end I sided with Xanathar's for just being such an indispensable tool for Players and DM's.

I like Monsters of the Multiverse just because I like all the different Race options. 2nd choice would be Tasha's for all the Class options.

I voted Tasha's mostly because of the flexible racial stat bonuses and also some spells and subclasses. But I love the lineages (Dhampir, Hexblood, and Reborn) in Van Richten's Guide to Ravenloft. They've got great features for both RP and min/maxing and great flexibility with two skill proficiencies. The Hexblood and Reborn are some of my favorite character options.

It depends on your role. If you're DM, the XGtE is best, if you're s player, the TCoE is better.

Shout-out to MotM for having so.mant races for PCs as well as a bestiary.

My question ersonal favourite is FToD, it just has tons of useful info and tools for running dragons. However, it's pretty niche and has very limited player options, so it doesn't compete with the previous three. Still worth a mention, though. 

Eberron has the Warforged, as well as the Changeling and Shifter, all of which have been hugely popular in my circles. I would estimate that 99% of games whose players have heard of Warforged, are using Warforged in some manner. It has lore that's more conducive to the way people want to make their own lore, with things like morally neutral necromancy, intelligent orcs, and magic item production as an industry, but hardly anyone is actually reading it and using it. Instead we're all out here reinventing it.

Ravnica has the best setting-specific player features I've seen, with the guild membership system and magic items, but the setting itself isn't that popular for whatever reason. And the really good stuff doesn't make as much sense outside the setting tbh.

I think the quality of the two Everything books is very slightly below these two, but since they're more universally useful, most folks are going to score them highest. But me, I vote Eberron.

I went with MMM because I think it is probably the best book to purchase beyond the core books of the PHB, MM, and DMG. While MMM did not really offer anything specific to me, as I owned all the races and monsters ahead of time, the consolidation of a lot of things into a single book is great for newer players. With just one purchase, a DM can drastically expand the options available to players and get a whole bunch of new monsters to use for themselves, making it probably the most bang for your buck you can get in a single sourcebook. 

I also like the streamlining of monsters they did.  5e’s monsters were a bit hard to use—you had to flip to other pages or click links to figure out what their spells did, rather than have all the information readily available at your fingertips.  That’s fine if you’re an experienced DM with the spell tables mostly memorised; it was less great for new or inexperienced DMs.  Even for experienced DMs, having the ability to play the monster more readily at your fingertips allows you to focus less on some of the mechanics and allows for better combat pacing. 

My favorites aren’t here; I really like VGtM and MToF for all the species lore and culture, but unfortunately that’s been tossed in the Legacy bin in favor of the bare-bones MotM. To be clear, I don’t mind having a collected and simplified master volume for people just looking for the stats, but it’s aggravating that they’re doing their best to quietly bury the previous works given how much more they had than just stats.

Despite there being a disclaimer claiming the oposite, Xanathar is a 4th core rulebook. It is basically a collection of everything that should have been in the core rulebooks but couldn't fit.

Quote from Dindomir_Eagleshield >>

Despite there being a disclaimer claiming the oposite, Xanathar is a 4th core rulebook. It is basically a collection of everything that should have been in the core rulebooks but couldn't fit.

 And that's why it's such a worthwhile purchase. I really hope that the options and rules from XGtE make it to the One D&D PHB/DMG.

Quote from The_Ace_of_Rogues >>

My favorites aren’t here; I really like VGtM and MToF for all the species lore and culture, but unfortunately that’s been tossed in the Legacy bin in favor of the bare-bones MotM. To be clear, I don’t mind having a collected and simplified master volume for people just looking for the stats, but it’s aggravating that they’re doing their best to quietly bury the previous works given how much more they had than just stats.

 Yeah, it's a massive shame about both VGtM and MtoF. I've skimmed through some of a friend's copy of Volo's and the lore and setting information is awesome. Still, it's nice to have everything compiled into one book (MotM)