There are many different repeatable bosses in D3. Some of them are easier than others (Belial isn't hard to defeat if you know the encounter and dodge his attacks).
So what bosses are probably good for bossruns?
There are many different repeatable bosses in D3. Some of them are easier than others (Belial isn't hard to defeat if you know the encounter and dodge his attacks).
So what bosses are probably good for bossruns?
Farming bosses is pretty useless in D3 - you only get guaranteed rare loot the first time you kill a boss.
Besides that, I think none of the bosses is actually hard if you know the mechanics.
Farming bosses is a great way to end a game after collecting a full stack of five Nephalem Valor buffs. This is due to the increased magic find and the guarantee that you will get extra loot drops from having active buffs. The group I normally run with will start an act a few quests before the final boss to get the loot from killing champions and boss mobs, then finish the game by beating the act boss. This nets a huge amount of loot and allows a great stopping point for checking the auction house.
Source: Diablo Wiki and Playing Experience