I was reading through ‘The Monsters Know What They’re Doing,’ and I was won head over heels by the Nycaloth.  It has Mirror Image, Invisibility, and Darkness at Will; really great HP and AC for it’s CR; and it can teleport in place of one of it’s attacks and not as a full round action.  This thing is a tactical nightmare for Players, able to employ hit and run tactics with near impunity and hardy enough to stand toe to toe.  Get two of them on the battlefield and one casts Darkness while the other casts Invisibility so it can leave the Darkness without the PC’s even realizing.  And did I mention the Blind Sight?

Name your own and why!  Stats, Lore, history in the game, fluff, pure appearance?

I suppose it's a litle pedestrian compared to the Nycaloth - but I like the Goblin. 

Now, I can't leave well enough alone, so the goblins of my games aren't much like the ones presented by the rules. But even if they were, goblins just do everything well. They're good at everything! Except, perhaps, heavy lifting. They're good at agriculture (with, perhaps, a preference towards animal husbandry - and btw, the rules mention rats and wolves, but forget all about spiders). They're adept tinkerers, leaning towards traps and alarms, but clockwork is clockwork, and they can build most anything if they want. They're well adapted to their role as villainous little bastards, and happily fill the cracks and crevices of human society. Goblins may not run banks, but if you're looking for a cheap potion, or poison, or meat of dubious origin, or simply to pawn your grandma's silver or a diamond you've lost the receipt for, then the goblin community is your bestest pal.

The things I've put my players through, using only terrain, traps and goblin archers. 

Goblins, to me, are comic relief with a purpose. And I truly love them the most.

I know it's been done to death but for me it's kobolds, for many of the same reasons as for goblins (see above). Kobold ambushes with traps and crossbows are one of my go to traveling encounters. You can scale them up with more dangerious traps and/or poisoned bolts but even the lowest level parties can typically survive a surprise kobold encounter without disrupting the main campaign in a significant way.

One of my favorite kobold encounters was when I DM'd a one-shot back in 3e that involved rescuing a local peasant who had been snatched by kobold raiders as a sacrifice to Tiamat. Players had their expectations set for sneaking into the kobold lair and facing off with a cleric of Tiamat as the big bad for the scenario, only to discover that "Tiamat" was in fact just a chimera that had tricked the local kobolds into believing it was Tiamat reborn (dragon with multiple heads... yeah checks out). Players had to completely revise their tactical plan on the fly. Ended up being a lot of fun creating the kobolds' defenses and then watching the players think their way through them so they wouldn't alert the guards. Watching them work together to crowd control the kobolds while facing a main threat that could fly was another fun part of DMing that scenario as well.

Ooh, mind flayers are great too. I use them sparingly - I think it amounts to twice in 20 years - because they're overexposed just like dragons (which I never use anymore). To me, it's fine that they have impressive abilities, but I find it much more interesting that they're such a foreign ... biology, mindset, everything.

my first option would be homebrew, but that's hardly an answer really!

Favourite monsters would probably be faerie dragons. They have the ability to cast a huge amount of mischeif, and they can build mini-stories as side plots. I once had a pair of faerie dragons use dancing lights and minor illusion to make a glowing figure who was calling for help (illusion for the sound), to lure the party into the woods. There they found some goblins, well below their level, and seemingly surprised that the party arrived. After the party killed the goblins, the faerie dragons swooped in and stole a shiny spoon from one of the goblins corpses - it was all they were interested in!

Kobolds. ‘Nuff said.

If I had to pick one - and that would be a difficult choice - I would go with the humble Flumph.  Floating jellyfish with adorable little faces that are also mood rings?  They are emblematic of some of the absolutely ridiculous designs from the late 70s, early 80s - so much so that later 1e products banished them to the “Plane of Silly and Unused Creatures” and 3e and 4e used them for April Fool’s jokes.

I find it delightful they are now a proud member of the Monster Manual, standing alongside dragons and other dungeon denizens.

I know dragons have become overdone in fantasy, but I have to say that my favorite is probably blue dragons. I love all of the chromatic dragons for different reasons (even red dragons) but I find blue dragons to be the most interesting. I love how the blue dragons are more tactical than any of the other chrom dragons and they have a very interesting social dynamic (they have one older blue dragon that acts as the leader of the blue dragons in the surrounding area and it is his or her job to approve marriages and decide on rules for the other blue dragons to follow). They are surprisingly civilized for dragons and they aren't neccesarily "evil", they just see smaller creatures as food (regardless of how intelligent they are). Displacer Beasts are a close second, I've always thought that they felt alien in a way that mind flayers don't (no offense to the mind flayer fans in this thread). If you're interested in learning more, check out the video linked below.

https://youtu.be/JGqJlNyHjE4

Dragons - large and in charge!

Whenever I DM, I have to force myself to not include Mimics in every corner of the world. They're a ton of fun, have some pretty interesting lore, and there's just enough variety in subtypes that you can use some variation of a Mimic in almost any situation.

Mine is certainly the Mimic. I've had some fun adventure moments with the whole "look at this really nice bed, so comfy and sof- aaannnndddd it's a mimic..." moments.

When it comes to uncommon creatures, I do fancy the golems. Certainly we've all had that moment where the whole "that statue looks, wait... Why is it attacking me! Let me hit it with my sword (hit - no damage) *player looks at DM  🥺." 

The Hydra because this creature reminds me of the Venus Flytrap.

The Beholder because its existence is just... so strange.

The Spectator because it seems like a lesser dangerous cousin of the Beholder; but it only just seems less dangerous.

Hydra is an awesome one, great history too.

I can't decide on just one but...

I really like the Nothic. The crazed being who discovered something that shouldn't be known is fun and I like to imagine them acting like Gollum. Plus the ability to know a character's secret, that's fun. 

Succubus. I like to incorporate dirty jokes and lewd humor in my games.

Chain devil, 100%. Just look at that design, so sleek, so focused, so cool! Also being able to animate chains is a pretty scary ability, they basically just get 6 attacks per round. With an average damage of 11 per attack, that's 66 damage per round. Honestly that reference is a little bit shoehorned but I'll accept it. Unnerving mask is a cool ability too I guess, I mean it doesn't really add much to the aesthetic but it does serve to make the chain devil more intimidating as a foe.

Sidenote, here's a super evil strategy. Place a chain devil in a room covered in iron chains, just hanging from the ceiling, piled on the ground, you name it. Have the devil use animate chains, then have it take a short rest and use it again. The chains don't vanish under any circumstances other than the devil dying or being incapacitated, which means they'll stick around after a short rest. So basically, wait 6 hours and you'll be able to have the devil make 26 chain attacks. So the players enter the room with this souped-up chain devil, and it immediately attacks. The party fighter is ripped to shreds within seconds, before everyone else follows suit because, wow, that's way too many attacks per round. Congratulations, you can now sit back and watch everyone file out of the game room in frustration, knowing they're probably not gonna talk to you again.

Chain Devils are very Hellraiser

Sorrowsworn. I’ve always though it was cool to have a monster that was, on some weird level, you; and sorrowsworn come closest to that archetype in current D&D. Combine that with amazingly cursed designs and mechanics that add some considerations beyond “hit it until it dies”, and you’ve got something special. I’ve been thinking of ways to expand on them, including ways to “personalize” them for certain PCs or NPCs, and filling the CR gap between the wretched and the other types.

i love a nice pudding, ooze, or cube. sometimes just shifting around the remains of previous battles and provoking unexpected perception checks, other times so slow and full of coins. when players become cautiously optimistic as they delve, out comes ooze's cousin the brown mold hazard. yeah, get that torch in there and show it who's boss!

one day i really, really want to build an adventure around an oblex and a alkilith squaring off in secret (but increasingly messy) battles across a town that is slowly being consumed without seemingly depopulating.

White dragons mixed with a blizzard and travel mechanics. They make a great monster where surviving them is the real test rather than defeating them.