Brute 750 has the most power for sure.
Kingquad and Grizzly will give you a better ride and are fairly even matched performance wise. Kingquad has a little more giddyup. Overall I would lean toward the Kingquad for the best all-round.
The Brute's had a few idiosyncracies in the beginning. The airbox intake prone to take on water too easily. Unsure if they have fixed that completely yet. Also the first 2005 models tie-rods and ends were too weak. There was a recall for them so if you end up going for the Brute, make sure the previous owner had the recall taken care of. Or at least had beefier rods and ends installed.
Good luck!
EDIT: Nice review of the Kingquad 700 when it came out.
http://www.atvscene.com/machines/suzuki/kingquad-05.htm