Is it Malgus' powerbase at it's height in FE? then easily Malgus, sorry but Greivous and the CIS might have been powerful, but they wouldn't be strong enough to take everything in Malgus' arsenal.
Arrrrrgghh! Star Wars jargon! What is FE?
As a response to what everyone has said, I agree that Grievous's forces greatly outnumber Malgus's, but I wouldn't dismiss Malgus entirely. Malgus's fleet is largely superior is Grievous' in terms of size, firepower etc.