With armies, police forces or one on one? If it's a military battle then Winston, I'm thinking WW2 was probably better training than the Falklands. If they're allowed cops in a totalitarian regime vs totalitarian regime skull-cracking grudge match then it's got to be Maggie all the way. Churchill just didn't have it in him to put in the boot like Maggie did, and he'd soon find himself detained at her majesty's pleasure on some trumped-up charges after being given a thorough kicking on his way to the station. One on one is a tough call, but my money's on Maggie. It'd be a brutal fight but despite the fat man's greater stature he just doesn't have the iron lady's psychotic killer instinct. He could make a bloody mess of her but she'd still be found ramming his own cigar into his eye or impaling him on a stiletto.
Brendan Behan vs Oliver Reed. Barroom brawl in neutral territory, both drunk as lords.