To be fair, OP doesn't sound like a person who is going to learn to play a class optimally (because currently there is no class that is easy at that level). If you want to relax as much as possible WHILE PLAYING AT A GOOD AND ACCEPTABLE LEVEL I recommend BLM. Once you learn the encounter it is pretty easy and there are few times you are going to ask yourself "what should I do next...". However, this is only true with knowledge of the encounter, otherwise it is extremely difficult to dish out good damage.

If you just want to do Dungeons and don't get vote kicked while AFKing though, I recommend Summoner...