One that treats you well, and buys you flowers when you're sad and brings you soup when you're sick!
In my opinon, there aren't too many not "nice" penises. I personally like them to be uncut, maybe a little rosy in colour, but not bright or anything. Being a good size works well, where they're not huge and not tiny. Erect tends to be nice...
As long as it looks healthy and clean, then it's most likely going to appeal.