We played Brain Age on the Nintendo DS, and enjoy Brain Toot, but the best brain games we found feature the dual n-back game that has an actual research finding behind them to influence fluid intelligence. IQ Boost and Neurosnack are the two current ones at the app store.
I have IQ Boost and it's excellent in clean professional polish and progressive statistics. Surprisingly for a 'clinical' game, it plays well and I find daily training worthwhile and fun. A bit on the expensive side, but well worth it!