TBC was easily the best for me as a progression raider. Had the most consistently interesting raids, always felt like there was something to do. Fuck Gruul's Lair and Mag's Lair though...ZA, Kara, and BT are still some of the coolest raids only rivaled by Ulduar and killing Cazic Thule for the first time.
Vanilla was extremely restrictive with class freedom. Druid was my favorite class but an awful tank (my favorite role by far) so I didn't do much besides run dungeons with some friends.
WotLK had brain dead easy heroic dungeons (dungeon running with my group who had played together for years was my favorite part of the game), boring zones, and a terrible lull in raiding content between Ulduar and ICC that nearly killed my raiding group as well as the pieces of shit that were ToC and EoE...of course, TBC wasn't exempt from these problems but it didn't bother me as much. Ulduar is still my favorite raid of all time though, sadly it wasn't enough to make me adore WotLK like I did TBC but it's still a strong second.
Cata tried to appeal to both casual players and more hardcore players but failed miserably. Was too easy for loons like me who played it like a second job and was too hard for people who just wanted to see content. My group cleared out every heroic dungeon in the first week of release quite easily. Don't think we had more than 5 or 6 wipes the entire time without using any form of guide. Blegh, distant fourth.
MoP just wasn't made for people like me. Dungeons that are only interesting the first time through and insanely easy raids. It has awesome zones though. Probably my favorite in WoW. It also killed my favorite class/spec which was my bear druid who had been my main since the beginning of vanilla (I have something like 320 days on him, totally worth it). But it also gave me my new favorite class/spec of brewmaster monk so I won't hold it against the expansion....much. MoP is pretty well done but just too different from TBC for me to fully enjoy so rating it is something I won't bother with.
