I don't know the abilities of any of the faction bosses, except now I've learned those of the ones I killed, but I would think that the tauren and blood elf ones should be the easiest, or at least the least defended, maybe sylvanas too, thrall is usually well defended, like stormwind and ironforge.

I didn't try either of those two since it seemed like alliance had started to wake up.

I'll see if I can get round to trying them if I find myself up at a suitable hour someday soon, at least after i finish the rest of my pilgrims.

I think the pally soloing varian was made impossible by a patch shortly after it happened.

My fights seem like they took about 20 minutes.