I use Dunlop Jazz III Tortex (the purple ones) because they're so damn grippy. I switch between that and sometimes the V-Picks, I use the medium pointed but I'd like to try out the smaller models soon.
Mr Petrucci is bringing out a signature pick soon so you just know I'm going to be trying those
I'd say just to go to your store and grab 10 or 20 different picks and see what you like. I think Dunlop do some type of variety pack which includes a whole bunch of their different picks.