The DS is my all-time favorite system. I remember first buying a DS Phat in 2005 after getting my first job. I bought Super Mario 64 DS and Polarium with it. It gave me my first experience importing games: Both Ouendan games, and Gyakuten Saiban II, so I could play Phoenix Wright: Ace Attorney-Justice for All a few months ahead of the American release.
Then in 2006, the DS Lite was announced, and I wasn't sure if I wanted to upgrade. Then I saw a Best Buy demo unit with New Super Mario Bros, and the difference the brightness made was like night and day to me. So E3 came and went, and the only American color coming was white, so I became impatient and imported the Ice Blue DS Lite. It lasted me a good year until the hinge broke. Sent it in to Nintendo, and since it was an import, I got a replacement unit. They offered me a choice of colors, including the then-rare blue-and-black and red-and-black. I chose pink because I like picking unpopular options. I still have it to this day.
Notable games in my collection:
-Castlevania: Dawn of Sorrow: Back in 2006, I had only heard of Castlevania in passing, but one night I had a dream that I bought Dawn of Sorrow, which came with this thick book that not only was the strategy guide, but also had a detailed history of the Castlevania timeline. I got the GBA Double Pack (Harmony of Dissonance/Aria of Sorrow), and fell in love. Because of DoS's rarity, I had to buy from Konami's website. Also, I love the other two DS games, say what you will about Portrait of Ruin.
-Clubhouse Games: Learned how to play a few card games on here, some better than others. Also completed my first game of Solitaire with it.
-Elite Beat Agents: Rarely has a game gotten me so excited and hyped to play. After playing Ouendan, I wanted that Americanization announced at E3 2006. It didn't matter what people said about the soundtrack, I still loved it, and like Guitar Hero (mentioned below), it introduced me to a few new songs I enjoy.
-Guitar Hero On Tour: Decades: I didn't want to invest the money and space into the bigger instruments for Wii, so I got this instead. Despite the cramps and limited tracklist, it was my first Guitar Hero game, and it formally introduced me to Journey, among other songs and bands.
-Mega Man Star Force 3: Black Ace: Many people dropped the Battle Network series after 3, but I kept going, and continued into Star Force. Despite bludgeoning the Power of Friendship over your head, I did enjoy the series, and 3 is the best of them.
-Pokemon Diamond: It took me about a year to beat, and it signaled my fatigue with Pokemon, even though I went and bought SoulSilver afterwards. I took a break after a while with Black, but came back with Black 2, and I'm still excited about Alpha Sapphire.
-Super Princess Peach: This game could easily have been shovelware, and despite being stupidly easy, it's still a lengthy, fun platformer.
-WarioWare Touched: I just bought it because it was rare. Still a good game though.
I also have a photo to share: My DS multi-cart cases I got from Club Nintendo (the one with the styluses has the Animal Crossing cover with the fireworks), my DS Lite and 3DS, and the box for my Ice Blue DS Lite.
I'm at a point in my collection where im down to the questionable titles that have little feedback to mixed reviews. I just picked up puchi puchi virus and prey the stars, i found both enjoyable and worthwhile additions to my library. 
