Most of this is based in TBC (My fav expansion) but that is also some of my most memorable WoW moments are from there. There are other bosses I really liked but I'd take too long mentioning everything.
Start - Kalecgos (Sunwell) = Single-handly ruined most of my summer (bad summer for recruitment and raiding), however he is a fantastic first boss.
Mid - Some very memorable: Reliquary of Souls (BT), Teron Gorefiend (BT), Shade of Aran (Kara) Yorsahj HC (DS), Alyrazor HC (FL), Professor Putricide (ICC)
End - I can't choose one but I loved (and a little bit of hatred) all of the main TBC end bosses: Kael, Vashj, Illidan, Archimonde = all of them probably gave me some of the biggest yells of any raid boss when they died.