If you like to roleplay Mecha anime, Mekton Z is the way to go. Its built specifically for that style/genre of play. You need the Mekton Z+ techbook to get really crazy with the mecha design, but with that and the main rulesbook, you are pretty much ready to go with whatever mecha series you want to recreate.
WARNING: The characters in MZ aren't nearly as detailed as the mecha are, and such things as Superpowers/Psionics/Magic that often goes hand-in-hand with many mecha anime are missing. There is a (very) basic Psionics system in the MZ+ book (techbook) but it needs a lot of work. Thus, if you want to simulate something like Five Star Stories or Giant Robo or Dangaioh, where the characters have superpowers, you are better off using BESM, The Hero System 5th edition, or GURPS.