Out of the ones I've played, it has to be Pokemon Dash. That game was... pretty bad. Most of the spin-offs I can replay. But that game... it just lacked any depth, and wasn't very rewarding either. It just seemed like such a gimmick game just to get Pokemon onto the DS.
Also, I have to admit, Pokemon Channel is pretty tedious, and truthfully, I only bought it for the Jirachi. xD Oddly enough, I can replay it, but that's probably because I wanted more Jirachi's. It is incredibly short, and not very exciting. I do have fond memories of classmates asking me for Jirachi's though (I was cool because of that XD), so I probably am a bit bias.
Also, I must admit, I'm not a big fan of the Mystery Dungeon series either. I even prefer the Pokemon Ranger games to it (I bought all three. ^^) I've only played Explorers of Time, and it seemed so repetitive, not to mention i didn;t like not being able to evolve until I beat te game. Other than that, I don't think any of the others have given me any issues. :)