My current girlfriend has the best body I've ever been with. Athletic and former gymnast, but still solid C cups...mmm...
As for hottest, probably my university girlfriend who was the hottest freshman in her year. I don't know how I ended up with her. Now she's married and has let herself go.
Fattest...yikes. One of my former roommates (we're talking twelve years ago) and I hooked up one drunken night, and the only thing I could remember is that she was big enough to actually be suffocating me when she was on top. That was the night I established one of my cardinal rules: I never hook up with a girl who weighs more than I do.