What's your favorite low-level monster (from either edition) and why?

[Yqatuba]

I've always liked the hobgoblins (even before I started Pathfinder and just played DND). They remind me of the Spartans, i.e very warlike and disciplined, and tend to live in city-states.

[VoodistMonk]

Kobolds and Mites. Pugwampi are way up there, too. Oh, yeah... the "why"... because Kobolds, especially in PF1, are a favorite of mine. I know how to use them, I know their potential, and I know they are fun to fight... I approve of their shenanigans and enjoy fighting them. Mites are really fun in much the same way. And the Pugwampi Unluck Aura is brutal at low levels [unless you are a Shaman's Apprentice Half-Orc with Sacred Tattoos and Fate's Favored].

Or swarms... of anything really. I love swarms at low levels... they're scary, and you have to get creative.

[Mark Hoover 330]

OMG mites. Defensively you've got DR 2/Cold Iron, high Stealth, Small size and 13 Dex. They get no great shakes in offense, though they can command Vermin. But that's not why I like them.

Goblins and their kin, orcs, even kobolds to some degree are a threat in a fight b/c they have been made that way, but to the exclusion of other utility. Some of these might have decent Stealth but they're very basic spies or thieves.

Mites though are the total villain package. In combat they have a defensive edge and might be directing their Vermin in battle, but out of combat they've got a Climb speed, Scent, 90' Darkvision and Perception +5 ON TOP OF that high Stealth skill. What's more, they get Sleight of Hand for distractions or entertainment as well as stealing from the PCs.

Then... mites are Fey type creatures. Want to change up your mites? What about some that don't take ranks in Handle Animal and Ride, instead spending these on Disguise and Knowledge: Local. While they wouldn't be super amazing at these skills, these would let them get by enough to slink around mortal settlements.

Want to add class levels or HD and make them more of a threat? This is a CR 1/4 monster with a 13 Dex and Wis to start from. They're never going to deal tons of damage through weapon attacks, but mites are a good chassis to build a lot of interesting foes from.

Mites weren't built just to fight with. Even at Int 8, these are thinking, reasoning foes; they have utility in and out of combat; they have just enough defense that they might survive being surprised, but their senses suggest it is difficult to do so. In short, mites are the monsters that know you're coming and who you are before you ever set foot in their lair.

[Sauce987654321]

Zombies and skeletons. It's easy to build them with any monster you like while still retaining a low CR/level. You can have large scale encounters with zombie krakens and skeletons the size of buildings while still being under CR/level 10.

[Dragon78]

Aberration- Otyugh and Rust Monster

Animal-Bats, Rats, Snakes, etc.

Construct- Soulbound Doll- can be used for good, evil, law, chaos, and everything in between.

Dragon- Pseudo Dragon and Fairy Dragon

Fey- Nixie and any sprite type fey in general.

Humanoid- varies

Magical Beast- Griffons come to mind.

Monstrous Humanoid- varies

Oozes- any

Outsiders- any Elementals

Plants- Leshy

Undead- Skeletons

Vermin- Any "giant" versions.