Not all of these are the best games available for the DS, but they are all games that I have enjoyed quite a bit. The DS has a massive library of good games, so it was difficult to assemble an interesting list of recommendations. I decided to mention several games that didn't get a lot of attention or were not given sufficient praise, though I couldn't resist listing my absolute favorite as well. I've also chosen not to mention any of the many remakes on the DS, but many of those are also quite good.
#1 · The World Ends With You
The World Ends With You's two screen battle system is overwhelming at first and it dumps you into the plot without enough explanation of what is going on, but once the plot gets going and you figure out how to balance battles few games can match it. Every bit of text in the game, including even the confirmation message for deleting your save file, is filled with clever writing and humor. The game has more good gameplay mechanics than it knows what to do with, and every RPG core element, from equipment to battles, has a new spin on it that simply makes it better than before. All told, TWEWY is one of the most innovative and polished RPGs ever created for any system, and no other game takes advantage of the DS's full spectrum of capabilities as well as it does.
#2 · Mega Man Star Force
The Mega Man Battle Network and Star Force series has its ups and downs, but the games are far too often overlooked for being the fun RPGs they are, and the first Mega Man Star Force is easily one of the best of them. Unlike some other games in those series it relies more on solid core gameplay than complex gimmicks, and introduces a lot of fresh innovation that was badly needed after the end of the Battle Network series. On top of that, it has a truly good story. The broad scale of the story may be focused on an alien invasion of a world with fancy technology, but the bread and butter of this game is stories focused on the lives and feelings of likeable characters, giving each major battle an emotional weight that is far more involving than typical RPG fare.
#3 · Infinite Space
Infinite Space has its share of flaws, but few games match its ability to create the feel of grand space adventure. Its story, derived from tried and true anime space opera tropes, has its share of charm and intriguing mysteries, but the real heart of this game is found in the enjoyment of improving your fleet bit by bit. You start the game with a mere cargo shuttle and must slowly build it up to a fleet of powerful battleships and carriers. In the process, you'll gather a full crew of colorful characters, and you get to customize each ship down to crew quarters and individual weapons. This requires an unfortunate amount of grinding, but it is still very fun to try new ship combinations and tactics.
#4 · Suikoden Tierkreis
Many Suikoden fans disliked this game for what it is not, but it nonetheless stands as both an excellent DS RPG and a solid entry in the series. Certainly, it is hurt by the loss of several features from older Suikoden games. A six-character battle system and a better way to utilize its large cast in major battles would have helped it. However, it more than makes up for that with some great characters, solid gameplay, excellent music, and a story that takes the core concepts of the series back to their roots and adds to their mythos in interesting ways. It may not be particularly innovative, but it is still quite fun and well worth playing.
#5 · Retro Game Challenge
If you like simple, fun games, then it is hard to beat Retro Game Challenge. This collection of new games created in the style of famous games from the 80s combines the simple charm and creativity of older games with some of the improvements in game design from more recent years. The result is a group of games that are all far more entertaining than they first appear to be. Of particular note is Gaudia Quest, the game's blatant Dragon Quest clone that manages to come up with enough unique gameplay elements and clever twists on old tropes to become something surprisingly memorable. It is a real tragedy that this gem was ignored and the sequel focusing on SNES-era gaming was never localized.