Best Co-op Single Kart (Mario Kart: Double Dash!)
Mario Kart is a series that doesn't really require a gimmick to keep the series going it's always been fun and inventive with its basic formula but Double Dash added something special with its co-op single kart play. The immediate reaction is to question how well that would work since only one person could actually be driving the kart at any given moment, what was the other person supposed to do?
The result led to some of the best teamwork simulations in videogame memory you and your co-driver had to co-ordinate everything, from item usage, power-sliding, and kart placement (so the guy in back could punch any kart near them). Communication and teamwork were musts. But maybe best of all, it allowed multiplayer Mario Kart that didn't end with someone throwing the controller and storming out.
Worst Buying Toys (Skylanders series)
The primary gimmick behind Skylanders is almost a good one you buy toys in real life to unlock characters in the game, and from there the character's stats, XP, etc. will be tracked wherever/however you continue with the series (including cross-platform). The problem, however, is that you are forced to buy each additional toy to unlock a character, turning the game into something of a neverending collection competition that can get mighty expensive very quickly (some rarer figurines have been selling for hundreds of dollars online).
The bigger problem with this is that the games are actually pretty fun and engaging, making the gimmick all the more frustrating. A lot of people love this it combines a nerd's love of obsessive collection with videogames. But one of the great joys of videogames is that they're separate from those kinds of money-dependent obsessions (imagine having to pay for each individual Pokemon). What the toys ultimately amount to is overpriced and not particularly useful DLC. But hey, at least they give you a toy. A toy would've helped soften the blow of paying for horse armor.