I would test all of em and give it one month on each.
For me Unity is more powerful has a great community and you can use C# code in it. Can be let down by 6Gb download to update to latest features which also kill off things they choose and deprecate commands you used a few months prior. High requirements. Must use Visual Studio now that MonoDevelop has gone so that size has to be factored into it.
AGK has quick build times, small downloads but isn't supported by big companies like EA (they have a unity licensed etc).
Unreal is more based towards C++ than a easy to use language it's probably the most powerful of the lot. Also has large download sizes and requirements
Take into consideration for 2d you don't need power just the ability to display what you've drawn where you want on screen. Anything even Java can do that, but for quickness AppGameKit gets my vote. Unless you want to be lugging around a PC it's AppGameKit I use it on a Jumper ezBook2 which Unity and Unreal struggle on.
-EdzUp