The Romans, definitely. Modern weaponry confers an enormous advantage but there's no way to defend against being overrun by a crowd 2000x bigger than you.
Well, it really depends on the circumstances. Are they in an arena? Because if it's just a 1v1 war then the marines could wreck them via airstrike or something like that. Also, it's not impossible for one man to take on around 200 with inferior weaponry. Just look at
Rork's Drift, where the British held off against a force that outnumbered their fighting men 40-50x over. In the case of a marine VS a group of people who likely don't even have metal armor (it was very expensive--you were expected to bring your own) you could shoot them down easily from a vantage point. If they were in a swarm-able position like at the bottom of a valley they'd lose for sure, but if they were in a secured fortress I think the Romans would have an EXTREMELY difficult time.
Also, you know, they'd probably surrender as soon as the dudes start firing magic fire arrows at them at 500 shots per minute.
