Ahhh... so many to chose from. So many different ways to ki.. err I mean challenge the players.
My personal favorite in terms of style points is Dark Creepers - the old Fiend Folio type, not the sucky 3rd edition types. Real Dark Creepers are creepy little guys lead by even creepier leaders that blow up when you kill them. Drow are overdone. Everybody has Drow.
In terms of the most common monster in my campaign, honestly its probably wolves of some sort. You start off early in your career facing wolves. Then you meet wolves with the 'rabid' template. Then latter you meet awakened wolves with a couple class levels of ranger. Then you meet worgs. Then you meet dire wolves. Then you meet goblins mounted on worgs. Then you meet worgs with the spirit template. Then you meet advanced dire wolves with the half-fiend template. Then you are like to meet winter wolves with the multi-headed template. Somewhere in there you are likely to run into werewolves of various sorts. I just seem to find lots of reasons to put wolves in the game.
Rats are pretty good too. Especially huge swarms of them. Ditto bats.
Goblins show up alot too. Kobolds are great. I don't do orcs. Ever. Ogres. Trolls. Giants. Lots and lots of bugbears. Never Orcs. There is only room for one ugly stock humanoid in my campaign, and its goblins and thier goblinkind kin. They've just got more class.
And I got to agree with Teflon Billy - gargoyles (and margoyles) rock. Heck, if there is a gargoyle template out there, I'm interested. A dungeon just isn't well dressed until it has some gargoyles in it. It's not like going to a business meeting without a tie; it's like going to a business meeting without your pants.
Skeletons and creatures with the skeletal template (and creatures with the skeletal and fiendish templates) have the wonderful advantage of fitting into every ecological niche. They are the go anywhere monster. Shadows terrify my players. Incorporal creatures are nasty.
I believe that every group should have to face yellow mold and green slime a couple of times in thier career or else they aren't well rounded dungeon explorers.
Monstrous spiders are pretty common in my campaigns too. I'm thinking that the whole wolves+goblins+spiders thing is probably from reading the Hobbit too much as a kid.
I love Griffins and hate that they are reduced to mere horse thieves in standard D&D lore. So much more can be done with them. In fact, I'm pretty much infatuated with all the heraldic type beasts. I'm much more likely to go nuts with an advanced Wyvern or Manticore than I am to have a dragon be the local monstrous tyrant of the area. I tend to be sparse with my dragons because when I do want the characters to face a dragon, I want it to be a _DRAGON_!
In the plant category, I think I'd have to go Yellow Musk Creeper. It turns you into a zombie. How wicked cool is that?
As far as fey's go, I like Buckawns. Fey with attitude. Let's dispense with this namby pamby pull thier hair for laughs fey. This is the spit you and roast you over a fire for laughs sort of fey, but with just enough nice to give the players a shot at talking thier way out of it. Any sort of fey like that will do. Quicklings are cool, but they are so seriously brutal that its hardly fun to use them except on parties getting big heads.
Mostly though, I'm a fan of making my monsters unique. I LOVE TEMPLATES. It seems like after a while, just about everything gets a template of some sort. If it doesn't have templates and class levels, its almost certainly a mere minion of something that does.