In my opinion there are a select five that I must say are absolutely a must have over all other's.
1. Chrono Trigger
2. The World Ends With You
3. Pokemon Black/White Version
4. Golden Sun: Dark Dawn
5. Final Fantasy IV
Some other DS title's worth playing are:
1. Final Fantasy III
2. Kingdom Hearts 358/2 Days (alot of nice content)
3. Kingdom Hearts Re:coded
4. Pokemon HeartGold/SoulSilver
5. Pheonix Wright Series ( they have 3 )
Note: these are not listed in favorite order. They are just listed.