Renly would have won. He had the largest army, and only needed to take care of Stannis (who at the time, was among the weakest) and march on King's Landing while Tywin and the Lannisters were fighting Robb.

Would Renly have been a good king?

Besides, Renly has all the makings of a great king. He's charming, well-loved, intelligent, kind and yet still politically savvy enough to play the game of thrones. Had he not been murdered by his brother, he very well could have taken the Iron Throne.

Could Stannis have defeated Renly?

Renly does have overwhelming advantage here and in a pitched battle Stannis is pretty likely to lose.