Actually come to think of it - Nathan Fillion would make a great Butters.
Some glasses, hair cut, and the right wardrobe and he'd be terrific.
So would Robert Downey, Jr.  Of course RDjr could do a good Rudy also.

Downey would be perfect if he were tall enough,...but he's considerably too short. The Fillion Guy is not even close to Harry in any way in my opinion.

Jackman would be solid,...but i don't know if he could pull off Harry's special brand of snark which is actually hilarious.

To be honest i think the best guy for the part is probably an unknown.

But if i had to go with a "name" actor,...i'd have to go with Chris Evans or Ryan Reynolds. Outside of Downey, they are the only other actors listed here that have the acting "chops" and or wit to portray Harry as closely as possible to the Novels.

And though they are not exceptionally tall,...they are not short. With the proper shoes you could get them up to 6-4 or better.