Define "fun".
People often think Keaton was the best Batman and he practically hated it given he had to do it in a super uncomfortable suit, same goes for Bale (in Begins anyway). I don't see why neither Hamm nor Gyllenhaal wouldn't enjoy themselves in the role, but even then I don't really think that fun is necessarily the most important factor.
To be fair, in terms of eyes I've always been someone who preferred the idea of white lenses to the raccoon eyes. But even then, just take a look at a few of the manips of Jon Hamm as Batman. He can do intense, eye wise.
Also yeah, the fact that he's the human embodiment of the DCAU Batman is a pretty good reason for why I want him in the part given...well, DCAU Batman is pretty goddamn definitive for the best Batman there is.