The Forces/Time build is probably the best bet (you could probably make a case for some Prime and Life for investing into his suit or reinforcing his own pattern....but that's probably being to complicated where Forces/Time is probably all you need).

One thing to keep in mind is that Saitama has the power of a Master if not a Arch-Master...but he has the enlightenment/arete about how reality REALLY works of a much, much less enlightened mage. Though if your just aping him rather then porting him, it probably doesn't matter.