It's going to sound like heresy here, but I prefer the original, non d20 BESM.
Big Eyes, Small Mouth original version, is a fun, very simple game that is easy to learn and quick to play. The second edition added more layers of complexity by putting all the add on books and later rules additions into the core book, but you can ignore that if you want. It's extremely flexible, and and you can create almost any character you can imagine (I've seen a PC party including a transformer, a 50 foot tall giant robot, a rip-off of Shampoo from Ranma, a cousin of Lina Inverse, a mad scientist, and a Jedi Knight, all as starting characters)
BESM d20 is a good, flexible adaptation of d20, but it is significantly different from standard core so there may be some unlearning to do at times. It is also a little more complicated than regular BESM.