@Kei:
Hakkar is probably my favorite boss fight. Using his own minions to poison him is awesome.
Hakkar was fun, but Sunken Temple was one of the few instances I only ever ran once. The benefits of it were mostly in the quest chains (especially the Rogue Specific quest), and the drops weren't all that great considering the horrible maze you had to navigate. I didn't feel like doing that a second time; besides, at that point, I was leveling so fast I got too high for it quickly.
Razorfen Downs I didn't even do in a group… once I got higher level, I snuck into it and soloed the two bosses that dropped what I wanted: the stitched zombie and Aggemnar the Coldbringer... got exactly what I wanted.
I guess I'd have to say Deadmines was my favorite of the early world instances though I've yet to do Strathholme or Scholomance yet (mainly because no one does them anymore). Strathholme will particularly be awesome if I ever get a chance to do it simply because it was pretty fucking horrible how you had to kill innocents in the city as Arthas and then it showed them burning people's bodies afterward in the CS. It seems the best instance storywise.
You know what'd be pretty awesome? CoT version of the Silvermoon assault. Though it'd never happen because it'd be a Scourge Campaign instance, and there's no way in hell Alliance or Horde would willingly assist the Scourge.
Did Underbog for the first time last night. The Stalker is pretty fugly; it's a shame he didn't drop that dagger. It's a pretty damn wicked blue.
Any of you participate in the Battle of Ahn'Qiraj? I joined only about two months ago and I completely missed that amazing World Event. Was it as great as everyone hyped it to be?