The answer to this question is quite simple when one applies Calculatus Eliminatus.
Sephy already had his ass handed to him by Cloud. Thus, the evidence is clear that he would not win.
Cloud is a second rate angst filled teen. Crono's character development, in comparison, is wonderful. Thus, Crono would easily blow him away on one hand. However, Cloud has a giant sword. This is easily countered by Serge battling with an oar. Thus, the evidence is clear that Cloud would not win.
Crono battled a fiend that could consume the planet. Alas, as much as it pains me, Serge battled a fiend that could consume time. Unfortunately that is clear evidence that Crono would not win.
Serge, however, is a pansy who is willing to go on insanely complex journeys at the drop of a hat. Magus, on the other hand, has a forceful personality. He could easily tell Serge to stab himself and Serge would follow through, since he has the backbone of lime jello. Therefore, that is clear evidence that Serge could not win.
Therefore, Magus would win.