Best trash - Mirmiron trash, in Ulduar. The ammount of lol's I had because of that.
Worst trash - Besides SSC, Ultraxion's drakes. Annoying, unnecessary, and a clusterfuck of purple fire everywhere, if the tanks were not up to their game.
Best: Ulduar. The Mimiron trash pre-nerf, and also the first packs going to Vezax...had some stragglers get killed then the shadowblade (rogue) mobs shadowstepped from where they were pulled to the entrance of Ulduar and one shot those same stragglers again.
Worst: Ruby Sanctum. A "raid" where you spend more time fighting trash than working on bosses...yeah, no. Honorable mention to DS trash because it was so mind-numbingly boring...which means the trash fit right in with DS.
FWIW: I didn't raid much during vanilla hence no comments on trash there...but thinking back on it, there was SO MUCH TRASH in those raids.
Can't remember where it was exactly in Kara, but there was like a 30 minute lull of trash in between bosses at some point in there. Maybe after Curator? Not hard, just long. (Before being overgeared for it)
Suppression room in BWL and chromatic packs in BWL. Worst things ever.
Also C'thun trash for being the only trash ever to be a 45 minute clear that actually never respawned (thankfully).
Best trash? Best trash is no trash lol.
Best trash? Probably the snails in ToT. I love those things. I love using life grip on my friends to pull them directly in front of the snails. Sometimes I make it out of the way of the snail, sometimes the snail gets me too. Even then, totally worth it.
Just to be mean I'd like to see Blizzard put a single stealthed snail on the bridge before Horridon, preferably with shadowstep. >.>
C'Thun trash, quad-Mindslayer variety by far.
Worst trash: any unavoidable trashwaves (hyjal etc, Niuzao temple, violet hold etc.)
Best trash: TotC.
I wouldnt call it the best or the worst trash, but the packs before general vex in ulduar, if you died and ran back in the rogue npc's would shadowstep to the entrance and kill you again
Most time consuming trash: Sartharion, Ruby Sanctum. The first time we raided those instances, trash clearing could take 45 minutes or more, just for one boss. Mount Hyjal had the same issue.
Most purely aggravating trash: BLACKWING LAIR SUPPRESSION ROOM
Most hilarious trash: Gastropods. Flame lords in Bastion of Twilight were hilarious too. Anything where people can screw themselves (or each other) over, really.
Worst 'trash': Ultraxion. By far.
PEPE SILVA, PEPE SILVA
The killer snails in ToT, the other trash in that place is like wading through sewage, but seeing the "ZOMG! ALL YOU NOOBJOCKEYS SUCK #%^*%* IDIOTS!!!"-guys get oneshotted by snails is just so hilarious that it's a disappointment when people manage to avoid them and nobody dies...
Mimiron/Elegon bouncy funtime dudes.
Failsnails in ToT <3 So much love for them, I find it hillarious to watch people be one shotted by them in LFR!
Worst, is definately the Ultraxion dragon trash for me...yawn.
Worst trash must be Hyjal, though I never did on tBC, I can relate to the pain that was feeled back then. I liked Blackwing Descent trash the most, it was either easy or fun like dwarven kings. Not too much time-consuming. Totally awesome and well-balanced trash. Some trash feels too much and unnecessery now. I'd say repetitive and long trash makes it boring.
ToT has some good and bad trash designs imo. Jinrokh trash is fine, Horridon trash is fun, but a little long. 4 Spirits shouldve been there instead of 8. Council trash is a bit long. Turtle trash is boring. Megaera trash is boring too but thankfully mostly skippable. Jikun trash is again too long, though snails are the most fun Durumu trash is boring and unnecessery to be that much. Rest is fine I guess.
AQ to C'thun was just dumb though. Took a very long time with the anubis characters, the large packs of exploding statue guys that required mass mana drains to kill, mindflays out the ass, and then more and more of all of that. Bleh :P I think that just barely beats out the Suppression Room antics.
Good trash? Seems subjective. I guess Firelands, since you could ultimately pick and choose what you wanted to kill. Everything else is basically the same to me, it's not terribly exciting gameplay. Maybe the BWL mobs that had rotating spell vulnerabilities. Seeing huge (for vanilla) crits on my warlock always made my day :P