Without a doubt, the first game, which is funny considering the controls are just the arrows and the A button. I feel like with each game in the series it gets a little easier, and this is mainly due to the tempo (with the exception of Gamer in Game & Wario). Both Inc. and Twisted! share the same speed and are the fastest in the franchise, so they are up there. Touched! slowed it down slightly but it was still challenging, and then Smooth Moves put it to where it's at now, at around 120 bpm, which was used for D. I. Y. and Gold as well. I think that's why the redesigns of the games in Gold sometimes aren't as fun to play since they had to be slowed down to fit the current speed. But Inc.'s final boss is definitely the most difficult, with all of Wario's intro games always being at level 3, and the speed ups being faster than the usual, not to mention Wario's Adventure itself being challenging. I've been playing the games ever since elementary school and I still have trouble getting to the 3rd boss!