The games are all in different places in time. They CAN'T even fight each other. Zone can't time travel as there is no evidence of this in-game. The classic island below? Honestly, it looks like a hologram. Also, if you were to actually make it so classic health was equal with Zone and Era, it would be a close fight for all servers. Classic swords do 1/6 of the total damage, which would be equal to about 8 health. Throwing a bush or bomb would do 16 health. Also, the healing bot thing is very annoying. "We have healing bots so we can never die". If you want to count NPC's, how about iClassic has the giant slime? Over 1,000 death occur as a result of the giant slime. If you can't get back up, that's about half of iEra and all of zone. If you add the rest of the Classic baddies, Classic would win as the baddies keep respawning.
There is no real way to measure this though.