For DS:
Pokemon Ranger
Mega Man Star Force (a remaster trilogy is coming soon to PC and consoles though)
Dragon Quest Heroes: Rocket Slime
Ace Attorney 1-4 and Ace Attorney Investigations 1 and 2
Hotel Dusk
Last Window: The Secret of Cape West (hotel dusk 2)
Mario 64 DS
Ghost Trick Phantom Detective

Pokemon: The Pit is a roguelike dungeon crawler romhack for pokemon it's interesting and you can choose how many generations of pokemon you want to see.

Scurge Hive is a cool action game.

Sonic advance 1-3 are great imo but they have screen crunch issues.

Fire Emblem GBA games are really good.

Boktai 1 and 2, 3rd game requires a translation patch if you can't speak Japanese.
And there are many pokemon romhacks if you're into that, like Pokemon Unbound, Emerald Legacy, Glazed, Crystal Legacy, Team Rocket Edition.
Legacy of Goku 2 and buu's fury are fun ones I never see recommended