Brock Lesnar destroying Zach Gowen will forever be the most Heelish act and one of my personal favorite beatings in wrestling history. You got a once in a lifetime monster in Brock Lesnar who is vicious, ruthless and shows no remorse in the same ring with a scrawny one legged wrestler being bloody and beaten right in front of his mother.
I think Triple H in his Evolution days, Taker in his Ministry days, and Lesnar in 2003 were the most evil heels ever. Taker would kidnap and crucify people, Triple H would bash his opponent's skulls in with sledgehammers (Goldberg, Shawn Michaels, William Regal, then Randy Orton and Batista), and Lesnar would use his massive frame to brutally beat up smaller people (RvD, Cena, Rey, Gowen), which was pretty damn evil. In recent memory, Alberto Del Rio is pretty fuckin evil, Mark Henry is middle to late 2011 was evil, and of course, Viper Randy Orton in 2009 is probably the most heel character since Triple H, like punting old men, hurting women.