I've used Mamewah, Mala, GameEX, MAME32 and AtomicFE.
I break it down like this:
If you are using a PC monitor GameEX or MAME32 work great at high-res.
If you are using a standard res monitor, Mamewah, Mala and Atomic FE look great!
If you are looking for simplicity, Mala is your answer.
If you are looking for animation and a "coolness" factor, AtomicFE is a good choice.
I use Mamewah on 2 cabinets and Mala on one. I love Mamewah but it is an acquired taste as has a steep learning curve to get it going...
I like Mala ALMOST as much...and it IS much easier to set up.