Not sure if you mean 1) if Freddie was alive, which song he could sing or 2) which song he would have liked.
Anyway, if it's the first option, I'd go for the more up-beat or powerful tunes, i.e. Sunny Day or Say It's Not True. He could probably do quite a lot with Up as well. Although it's very hard to speak for an artist. If it's the second option, I think it's even harder, but my guess is he'd appreciate the beatlesque mood of Smile and could possibly take it to another heights if he was still here.
Sorry for the restrained tone of this message, but it's really hard to imagine what a guy like Freddie would like or do. He'd probably blow us away, though.