[quote=Aristo]
Depends on the system you have in mind. There are a few rulesets you can find online for free; Giant Guardian Generation and Zodia Sunset, while based on more of an anime feel, can easily transition to a more Battletech setting.
[/quote]

The simple d6 system, which is a generic ruleset applicable to anything. it's also really easy to learn, ridiculously uncomplicated, and easily moddable.