I'm probably the odd man out but worst trash was ToC... It was very boring for me to just go in and do bosses. Trash is like filler in a story, otherwise it's just "Once upon a time, the dragon king died. The end." I love trash!
Best trash, tough call. The trash in the blood wing of ICC was great because everyone was fighting to get that orb buff. OR! Best trash was the gold trash in Karazhan, everyone looked forward to that theater room.
Worst is probably corehound packs in MC, mess that up and it's a wipe or run out reset (which is sometimes a wipe if the imps have respawned).