Donaire, Pacquiao, James Kirkland. And even though it's P4P i'd still say W. Klitschko is up there.
And I know it's no longer an in vogue choice but...Randall Bailey anyone?
A couple years ago I would have put Arthur Abraham right up there but his punching power was lost with the eight pounds he gained :( .
Even though he is nothing but a right hand and was never a very good overall, yes he has the power to stretcher anyone anywhere near his size.
Alexander took those right hands quite well, Bailey lost some power cred in that fight.
Would you rather take a punch from these guys or a punch from this guy
http://25.media.tumblr.com/tumblr_ma1xq9uOV31qbzyr6o1_500.jpg
Hi, you must be new to boxing. Are you familiar with the term "pound for pound"?
Would you rather take a punch from these guys or a punch from this guy
http://25.media.tumblr.com/tumblr_ma1xq9uOV31qbzyr6o1_500.jpg
P4P, meaning power relative to your weight class/size.
Have to give David Haye a mention. He has insane power for essentially a very small heavyweight/blown-up cruiserweight. He wobbled Valuev, when being very cautious. He hits harder than most, if not all, heavyweights right now.