It'd be a violent battle, and... likely Saber would pull off the win (being Saber and all), but anyone else, Caster has decent odds of pulling it off.
Ignoring HF, Medea could have won in the other two (or at least made a better showing), but is the bad guy, so she has a lot of issues to deal with. Still, think her loss was being a touch impatient, a touch too showy (the draining energy from everyone in the city was an issue), and a touch too arrogant and/or naive.