Really depends on whose style you like ?
I've done most of my commissions with Frank Kadar whose service and work has been fantastic. Hope to do a couple more in a week or two. Same goes for Ashleigh Popplewell. Have bought awesome art from Meghan Hetrick as well, but it wasn't commissioned. Check over on Scoundrel and Blowout as to who is open for commissions and see who you like. There's also a useful feedback thread on Blowout that can indicate artists you might want to avoid due to them not delivering or being terribly slow.