#18 Advance Wars Dual Strike
Content! So much content! This game is almost permanently loaded into my DS Lite. For me it is the absolute best game in the franchise. Some might argue that the different CO-modifications make the game a bit broken. That might be true, but the level of strategy it brings makes up for it for me. The Dual Strike CO-powers are also over powered to stupidity. But I still love them.
A long campaign, hard mode, massive library of extra maps and time, turn and money challenges. I have probably spent around 400 hours into this thing and I have still a lot of things to do and master. Each CO had like 10 costumes each and I remember one day spending a lot of time picking all the outfits that I thought was sexiest for each character. Probably the only time I have done something like that.