Bosch G series is the most flexible panel, IMO. I base this on the my personal experience. Of course you need to be a dealer to get them.
Vista is #2 (its everywhere and the #1 selling brand in the world)
DSC #3 (ease of use, wide availability, price)
Napco, ELK, DMP, GE all a distance 4th.
Nothing against DSC (probably the only other real option) but with their "1 piece comes bad out of box on just about every job" issue which means on your first job you have to buy two systems so you can finish. They are cheaper and sometimes it shows... sorry...
Jim