Many people have had success with GameMaker Studio and many more people with Unity3d as well. Which you prefer depends on what your project requires and what kind of budget you have. As far as actual viable exports, I'm going to call them about the same. There are a couple of differences, but generally they both have the important stuff covered.
GameMaker Studio is more focused on 2d games. It has some minimal 3d capabliity, and can use custom shaders(OpenGL ES) for both the 2d and 3d. For 2d games, it makes things much simpler than other game engines, including Unity. The GML scripting language is also quite handy. You can do almost anything you want with it. The catch is that it is not useful outside of gamemaker. But is has similarities to C-like languages so isn't really difficult to pick up if you have any experience coding. The thing about GMStudio is that there isn't a free version that can export to mobile platforms. The free version has too many limitations to be useful. On the other hand, the master collection is normally $799. It contains all the exports, and all of the features it has for this one price.
I've mentioned these two engines, because they are the ones I know best. There are many other options, for example Shiva3d, and the UDK can do iOS, but not Android, at least not in the free version.