My favorite is Greater Cleave (and just old fashioned Cleave is a close runner up) there's nothing so satifying as hacking someone down, and then getting a prize--getting to hack the next guy too!
It's great when a wizard has fireballed a group of bad guys and you can step into the middle of them and chop them to pieces in short order.
Any human warrior type (Fighter, Barbarian, Ranger, Paladin) of mine always takes Power Attack and Cleave as his first level feats.