It's been a while since I crawled through dungeons or instances in an MMO (or jumped into Eve's wormholes or complexes, even) but I couldn't help thinking that Rift's 1-2 player instances, called "Chronicles", were a rather good idea for making the body of the game's instanced content available to players who really only want to play with a single chum. I certainly did quite a lot of WoW with just Alec or John as a companion, and we often struggled to be bothered to get a party together for the dungeons, because we'd always end up with some guy who ran into walls, or a companion who could only type "what". Anyway, it got me thinking about the Dungeon experience in MMOs, and I ended up thinking of two that I've enjoyed most: Guild Wars, for its largely instanced storyline, where pretty much everything lay within areas you'd play with a couple of friends, or even solo, and City Of Heroes, whose missions always seemed somehow "logical", thanks to taking place in underground bases, warehouses, and so on.
So I ask you lot: where is the finest dungeoneering to be had? Which MMO does it best?