Here are my favorite single player games on the DS, all of which took me some time to complete, because I wanted to get all of the extras.
Castlevania Dawn of Sorrow:
-- AWESOME game. My favorite DS game to date.
Castlevania Portrait of Rune:
-- Not as good as C: DS, but still a great game. It has its own strengths, it just took a little bit longer for me to get into it.
Zelda Phantom Hour Glass:
-- A bit too easy, but it's a GREAT game and would probably be my favorite if not for C: DS. It has lots of little things to do. I've been playing it for about a month now. I just got all of the items, so will beat it this week.
Magical Starsign:
-- FREAKING PLAY THIS GAME.

Especially if you like RPGs. It required a bit of time for me to get into and it's very odd at first, but it ended up being so enjoyable. I played it over a period of 3 months. I really hope they make a sequel.
Mario Kart:
-- I unlocked every bonus on this game and had a blast. But I think that since it's such a great multiplayer game is the reason I spent so much time practicing and unlocking the extras.
I need to mention Nintendogs. But prepared to be consumed if you get into this game. I had to put it down because it was taking up so much of my time. I felt guilty at first for doing so.

My weef also gave me some weird looks as I played this game for months and months and months...
Anyway there are soooooo many good games on the DS. I missed out on a few because I just don't have the time to play them all.
<]=)