Many people on this thread have mentioned 3 and 4 as their choices, but I just never could get into either one. 3 absolutely put me to sleep. 4 was a little bit better, but neither really reached out and grabbed me the way FFCC and FFTA2 did.
I mean, I have played FFTA2 over 220 hours and STILL play it every now and then. So many classes, so many weapons, all the regional auctions, trading items with friends via wifi, etc. etc. etc. The only other games I've spent anywhere close to that many hours on was FFCC:RoF (playing co-op with my 3 daughters=AWESOME) and Disgaea, which is definitely an underrated turn-based strategy game and should be mentioned in any "DS Top 10" list. Not to mention TWEWY, which might be the most cutting-edge, innovative title EVER released on the DS.