A few games that never made it to North America but did come out in Europe.
Bomberman 2
Released in Japan as "Custom Battler Bomberman", in addition to the typical Bomberman action you'd expect, this has a genuinely solid single-player mode where you get to customize your Bomberman with various permanent powerups and stuff. It's really cool.
Mr. Driller: Drill Spirits
This
technically came out in North America, but that release is trash, with missing features and game modes compared to the Japanese and European versions. It was presumably rushed out to meet the North American DS launch. The European release is essentially the best English-language game in the entire series. If you don't want to import, you can pick up the DSiWare release, Mr. Driller: Drill Till You Drop, on the 3DS eShop, but it's really just a scaled-back version of Drill Spirits.
Pang: Magical Michael
Ignore that box art, it sucks. You might remember the Pang games by their North American title, "Buster Bros." They were a pretty popular series of arcade games where you shoot bubbles which split in half. Magical Michael was the last game in the series before Mitchell Corp. went out of business, and the double screen height might just make it the definitive version of the game.
Honorable mentions:
- Zoo Keeper is bugged in its US release, if you close the DS it will just keep running, the sleep mode is completely broken. The European version is perfectly functional.
- Disney released several games with a weird pointless social network thing called DGamer. The server is long-dead, but every time you boot up one of these games it'll ask if you want to use DGamer or play the game. It's only a minor hassle, but it gets annoying if you play many Disney games. The European releases never had DGamer at all, so they're pretty nice.
EDIT:
Monster Tale was a pretty limited release, and is short and great. Worth playing for sure.
I'm still waiting on that 3DS remaster. Apparently the DreamRift CEO ghosted all the employees and they were never paid for their work, so that sucks plenty. My avatar is from their only other game, the 3DS Epic Mickey. They were a really talented group of people, breaking off from EA after the Henry Hatsworth game that's been recommended several times in this thread. It's unfortunate that we never got to see what they'd do next.