Best 'sport' bike for a tall rider?

Every model of motorcycle has a little different ergonomics, i.e. the spacial relationship between seat, handlbars and footpegs. How they fit you is entirely subjective. SIt on a few 250cc bikes and see how you feel.

My guess is that they will all feel a little cramped to you. But I could be wrong. Your weight is not going to be a concern, just your size.

Cruisers have an advantage for big guys because you put your legs out in front of you instead of folding them under you.

Highest overall seat height on a sport bike is around 33 inches on Yamahas I believe. At 6'4 you should be able to put both feet flat on the ground, which is a good thing. At 180lbs, sag shouldn't be much of a concern either. For a more upright ride check out Suzuki's DRZ 400. These bikes are sweet for beginners.

For your size, you will quickly if not immediately out grow the power of a 250cc bike. Go with a 600cc bike if you are truly looking for a motorcycle with a sport bike look and feel. Check out Yamaha's YZF-R6 and the FZ6R, you'll be glad you did.

Agree with smarty pants and would add that you are unlikely to be comfortable on a sports bike after at most an hour