Best BT Controller for Games

So...its not often that I play mobile games, being that many of the best ones are on iOS first and Android second. While I have an iPad, and gaming on it is great, at times the iPad can be a bit heavy and bulky.

The solution I've come back to is to simply use an emulator. DraStic is one of the best DS emulators I've come across. Playing a few games on my S4 has been a really nice experience...mostly.

Having on screen buttons is becoming a problem that I want to remedy. So I come to you all for advise.

What is the best Bluetooth game controller that you've come across? Preferably one that attaches to your phone and is a good candidate for using with emulators.

Thanks in advance.