[KaVir]

It is indeed completely subjective. Combat plays an important role in many muds, and people tend to stick with the systems they like, so I suspect the majority of players consider their favourite mud to have the "best" combat system.

You might find the responses more interesting if you try and move away from the more subjective criteria. What combat systems allow you to use each weapon in a variety of different ways? What sort of support do different combat systems have for ranged weapons, two-handed weapons, shields, improvised weapons, etc? What type of injuries and penalties do combat systems support other than hit point loss? What sort of armour systems are utilised by different muds? And so on.