The only one of those I experienced while current was Deathwing. If being current isn't necessary then Rag is my fave fight (experienced heroic kill while DS was well underway.) Arthas being runner up (experienced during Cata.)
Liked Arthas until the LK expansion and tie-in book entirely destroyed his story and made him a total pansy. Illidan had "cool" factor, til they made him a boss for no reason whatsoever. Deathwing kinda sucked. I'd go with classic-era Ragnaros, purely for it being the first raid I cleared, and he was a totally epic boss, even if, after a while, the raid itself made my want to chew off my own feet at the ankles.
In the book, it says He created us to be just like Him. So if we're dumb, then God is dumb, and maybe a little bit ugly on the side. - Zappa