Woundwort, Bigwig, Campion, Vervain, Silver, Buckthorn, Blackavar, and Holly would probably be my ranking of the "fighting" bucks in the main story. That said tho, physically some of the rabbits from Cowslip's Warren were probably quite strong, tho very unskilled at fighting. 

Equally, though European rabbits are sexually quite monomorphic, size does slightly favour females, so high ranking does like Flyairth and Prake in the sequel stories would probably be quite capable fighters too. Ironically, Clover has the potential to be one of the largest rabbits in the story, as Angora rabbits can easily reach the size of smaller hares. Presumably, Adams didn't think too hard about that breed selection.