Touch Detective (and it's sequel) Touch Detective 2 1/2, were greatly overlooked as two of the finest games for DS. IGN gave both games low scores which they clearly did not deserve. As you may have guessed by the titles, they are mystery games. They are very funny and have a lot of things you have to figure out. Plus finishing the touch list and investigation report takes quite a while. Hopefully you got all the touch items during each crime case the first time. Otherwise you'll be redoing those cases over and over again trying to find that one thing you missed. (Though you could always just look it up on a walk through if you give up.) Fortunately, this didn't happen to me in either game, but it certainly could happen to you if you aren't observant. Granted, most of that stuff could probably be touched in the bonus level, but there are some exclusive to certain cases. Both are very good games that I highly recommend. I even uploaded Sav files for both games!
Pokemon Ranger is very overlooked. It's certainly better than all the other crap Pokemon games for DS .Yeah, including Diamond/Pearl. I'm fed up with the Pokemon RPGs getting far too complex, and having far too many pokemon.
Pokemon Ranger is the best Pokemon game I've ever played, and it makes sense since HAL develouped it. (I love all of HAL's stuff (except Arcana)) True HAL was involved with Pokemon Pinball and the stadium games, so those are exceptions.
Pokemon Ranger has the best sprites I've seen for humans in any Pokemon game (in the over world that is (which isn't saying much)) and has the best story line of all the pokemon games, even outside of the DS. Really fun, great graphics, and it was an effective way to get me used to some of the bizarre Ruby/Sapphire Pokemon. Love that game. you must play it.
I'm not sure if it was overlooked, but Magical StarSign doesn't seem to get enough attention.