For me, I've beaten deity in all 4 of the "main" victory conditions with civs that are poorly suited towards that victory condition, so I have that bragging right. That said, I don't play the game for bragging rights but rather I play it because I think it is the best and most fun game there is! As such, I usually play Emperor - anything below and I'm just not seeing any competition at all, making everything from the medieval era on a boring "click next turn" snooze-fest. Anything above Emperor, and I don't have the unlimited freedom to do whatever I want and still have things work out the way I planned... So I guess my vote would be for immortal - to me, that's the first level where I need to sacrifice some of the things that I WANT to do in favor of things that I NEED to do.