New Super Mario Bros. - 3.5 - It's not the best 2D mario platformer by ANY means (okay maybe graphics), the level designs suck ass but it's still a 2D mario platformer so it's still good.
Mario Kart DS - 4.5 - Amazing game, really fluid graphics engine, great online play and overall solid gameplay mechanics; lots of replay value, still requires skill unlike MK Wii.
Castlevania Dawn of Sorrow; Portrait of Ruin; Order of Ecclesia - 5 - This series on the DS is definitely a must-have, all three games are amazing, have tons of replay value and offer a lot of content backed with addictive and deep gameplay mechanics.
Ace Attorney 1-3 (Phoenix Wright); 4 (Apollo Justice) - 5 - Word of warning here: if you do not like point and click adventure games that require a LOT of reading, a lot of patience, lots of paying attention and lots of logic and puzzle solving, then avoid this suggestion altogether. This is my favorite DS series along with Castlevania, the writing is superb, the cases are creative and engaging and it always takes a lot of logic to beat the puzzles (there are almost no idiotic things like in other adventure games like Monkey Island), the characters are extremely fleshed out and all have a distinctive personality of their own and the story has a genuine emotional impact, the music in particular, is just brilliant. Like I said before, it's not a game for everyone but for those that can enjoy it, it's amazing.
Pokemon Platinum - 4 - It's classic Pokemon, if you enjoyed the previous games then this is just more of the same; if you didn't like the previous game then skip this one altogether; if you are new to the series, platinum is a great place to start.
There are a bunch of other great games on the DS as well, Elite Beat Agents, Professor Layton, Trauma Center, Contra 4, N+, GTA Chinatown Wars, Henry Hatsworth, The World Ends With You, Ninja Gaiden Dragon Sword, Mario & Luigi Partners in Time, Final Fantasy (the remakes, the RTS based on FFXII and Crystal Chronicles), Chrono Trigger remake and a bunch of other great games.