I have a few really bad DSiWare games, like
Snakenoid,
System Flaw: Recruit and
WarioWare: Snapped.
I have plenty of DSiWare titles, but these three really stick out in my mind as the worst ones by far.
Snakenoid is incredibly monotonous and pretty tedious, and
System Flaw: Recruit and
WarioWare: Snapped! are just straight up broken and borderline unplayable.
Another downloadable title that comes to mind is the WiiWare game
Bloons.
Easily the most tedious and frustrating game on here. The goals are WAY too strict and hard to get, especially for a game like this (I can't even get past level 2!), and the overall gameplay isn't fun in the slightest. Even the free Flash version is WAY better than this pile of garbage.
One retail title that immediately comes to mind is
Game Party for Wii.
Without a doubt the worst Wii game I own by far. Essentially a minigame collection, but a bad one in every way. It's mainly hampered by it's controls. The only game that controls OK is the Trivia game (which is just point-and-click). All the other games control awfully. The controls are incredibly unresponsive in general, and range from too stiff on games like basketball to too twitchy on games like table tennis. Other games like ping cups and darts are frustrating because no matter what you do you'll never hit where you aim (darts often land on the other side of the board). To top it all off, apart from the games being bad, there are so few games in there that you'll be shutting it down and booting up something else in 15 minutes tops, because by then you would've tried out every game on there. Sure, the minigames have unlockable extra modes, and PAGES of Avatars to unlock (rather than, oh I dunno, USE MIIS), but who cares when the games aren't fun to play? When your minigame collection is so bad I consider booting up Wii Music again instead, then you know you've reached a low point. Definitely the worst retail game in my collection.