The Leapster isn’t great, although I’m frankly not buying “too easy”. There’s lot of mediocre platformers with Diego or whatever that are if anything too hard. But they also aren’t very good.
Having had first hand experience with a bunch of DS games with a 2-3 year old, there really aren’t many great games for that age group. You get to 4 and 5 there are a lot of options, but 2-3 the stylus and d-pad are too tricky for the little ones to master.
iOS is really far and away the best handheld platform for young preschoolers. The touch screen is intuitive and there’s a genuinely good selection of games. The iPad is the best given the size, but the iPod touch works pretty well too.
(Full disclosure: I’m a developer of kid’s games, and have a pretty successful iOS game for preschool kids.)