BESM d20 is one of the better systems out there as long as your players are all willing to do 2 things:
1) Whatever happens, roll with it. It's an Anime setting, after all, so you can go ahead and be awesome
2) Agree to reign themselves in whenever they have to
Given those two conditions (which are fairly common among tabletop games anyway, looking at you Paragon Surge Oracle...), it is ridiculously entertaining and has an integral and fairly comprehensive class-creation system (which was their basis for making the ACTUAL classes) for anyone who wants to do something a bit differently.