Of all the Flappy Bird clones littering the App Store at the moment, only a fraction of those games is actually playable -- meaning that they're fully functional and can be used without glitches, slowdown or outright crashes. Of that small group, there is but one that I've found that is worth your time: Ironpants.
Ironpants plays almost exactly like Flappy Bird with the only real difference being that you hold your finger on the screen (rather than tap) to get your tiny superhero to fly higher. You still need to navigate through narrow passages without touching anything; the game goes on forever just like Flappy Bird; and it's also punishingly difficult.
In fact, Ironpants may be even more difficult than the game on which it is based, and that's saying something. The tiny onscreen character moves a bit faster than the titular avian in Flappy Bird, and because of that, it's much harder to make adjustments before slamming into a wall. My high score on Flappy Bird is a respectable 51, but I can only manage a 6 on Ironpants. It's that difficult.
To be perfectly clear: If Flappy Bird were still on the App Store, I'd probably not recommend checking out Ironpants because it is indeed the inferior product. However, with Flappy Bird's creator promising to never re-publish the game after he removed it from the App Store, it's the only halfway decent alternative currently available.