Worst main-series game that I've played (i.e., not NSMB2, or Yoshi's Island if it counts) is probably NSMBU. Playing through the single-player, it was always either bland or gimmicky, with a couple of needlessly (and I'd wager unintentionally) hard-to-find secrets. Like 2-4's secret exit, which requires you to run backwards up against what appears for all intents and purposes to be a solid wall in an otherwise sparse area while small. Why?
Also, having played every console Mario Party through 9 and DS, 4 is the only one I wouldn't care to play ever again, because I hate the board maps, the mini-mega gimmick is annoying, and there weren't any more minigames than there were in 3. Personally, I like 3, 5, 6, and DS the best overall, particularly 3, as all of the boards are pretty enjoyable.