Hotel Dusk was one of my favorite DS games so I chose that. I even ended up importing the sequel which was never released in the US, which I believe is the only import game I've ever bought. Loved the art style, loved the story, loved the characters, especially Hyde. I own Radiant Historia but haven't really put any time into it, I got away from doing much handheld gaming about the same time I bought it but someday I plan to give it a full play through. But I can't imagine it will match Hotel Dusk as one of my favorite DS games.
I'm not sure if it can still be found online for a reasonable price, but the sequel to Hotel Dusk is very good as well, I don't think it's quite as good as the original but still definitely worth playing if you can find a copy of it.