Well assuming the guy takes care of himself I think gym shorts and a wife beater, sweats and a wife beater, shorts and topless, sweats and topless, sweats and and a t-shirt, you get the idea. Pretty much sweats and anything where I can see a guys arms...but I mean even if he was in a sweatshirt I'd probably find that hot too.
something that suits your personality and physique, looks comfy and relaxed.
I would venture that I think my favorite was a pair of super baggy, floppy sweat pant pj things and no top. and leave your hair a mess. think it might just be because it suited him and because I am sure he wasn't trying at all.