I'm proud to say I beat all three games without cheating. The first game is definitely the hardest, only because of the birds. I'm sure you remember the birds. You'd go to jump over a pit and a bird would come out of nowhere, hit you in midair, and knock you right into the pit. And if you actually managed to survive and were knocked back to your platform, if you tried to jump again the bird would reappear.
The second game I didn't find to be all that bad. It is definitely the best in the trilogy when it comes to graphics, story, and innovation. They fixed the reappearing enemies glitch, too, so that made this game much more playable.
The third game is my favorite, though, simply out of nostalgia. My brother originally showed me the game back when I was 7 or so, and we used to play through it together. I picked the game back up a few years ago and pushed myself through it. The game is a step down in the story department, as well as graphics (most cutscenes simply have an abstract blue background, instead of actual settings like in the other games), but the gameplay is glitch-free with a lot of new moves, so it's fun to play. Most people claim that it is the hardest, but if you take the time to memorize the levels, it becomes the easiest.