... is probably someone who enjoys pain, but since we don't have a specific Masochist class in WoW, the task of raid leading has usually fallen to anyone who is ambitious, has a lot of spare time, and doesn't mind being hated by large groups of people for no particular reason.
A lot of guilds I've seen have had one main tank/guild master/raid leader, mainly to prevent the problem of a tank taking all your gear and deciding that he'd really rather be fury/a mage/in a better guild. However, tanks have to spend a lot of time farming, which leaves little time for dealing with all the issues that come up in raiding guilds. Plus, a tank who is raid leading is likely to have a ... shall we say, very unique perspective on the action that lends itself to a lot of yelling at healers. "HEAL ME HEAL ME HEAL ME" isn't a great way to keep up healer morale.
A couple guilds I've been in, including my current one, have had raids led by resto shamans. Shamans, priests, and other healers who like healing seem to be pretty good choices to lead guilds and raids. Healers are used to taking a lot of abuse, and their perspective in raids lets them have a good idea of what's going on and what the problems are. The only downside is that healers can't really see the action, and if your main tank gets into a spat with a guild leader that also happens to be a healer, your raiding progress can go downhill pretty fast. Warlocks and hunters are also good choices for raid leaders and particularly DKP compilers, since they don't have any vital responsibilities in many raids and can alt-tab to check DKP and strategies without killing everyone. I knew a warlock who led raids while checking strategies, compiling DKP, talking on Instant Messenger and who knows what else at the same time. Even then, he has visible scratches on his wall from throwing the keyboard at it during raids.
I can't imagine being a rogue leading a raid, just because we have to spend so much time running around and can easily kill everyone if we're not paying attention. But in the end, any class can lead a guild or raid successfully as long as they're firm, patient, thick-skinned, selfless and willing to pour their heart and soul into the guild. It's not an easy task and I've never considered doing it myself.
What class do you think is most suited for guild/raid leadership? What class is your raid leader?
(Edited to add: I was incorrect in assuming that the guild leader is also a raid leader, and have changed the title and some of the text accordingly. The class of a guild leader doesn't really matter -- the class of a RAID leader might.)