My two personal choices would be SMS or NES. NES has a more active community (as has been said), but the arch is much more complicated and has more limitations graphically that can be quite challenging. But then again, there's nesdev forum where quite a few experienced members hang out and help out as well. NES has some decent debuggers too; more advanced than what I've seen for the SMS. A good debugger is worth a lot IMO. But the graphic freedom of 4bit pixels for sprites and tiles on the SMS is really hard to ignore (a very
nice plus on the SMS side).