It always depends on the context. Sometimes you wanna be funny, other times you wanna hurt the guy you're saying it to. There's no simple answer, IMO.
I have to say, though, that my LEAST favorite way is "I'll pray for your soul" or something like that. That's the "religious and thus morally superior" way to say it. No one who says that means that they're genuinely concerned for you, and I'm pretty certain they never actually do pray for you. It's just a way of saying "I'm better than you, so get out of my face, heathen!", ie "fuck off".