Tibianic= If you want to rpg play for long etc
Realots= If you like some custom things to balance vocs and the 7.7 client(wands,pz boats,hydras,etc)
Mastercores= Abit higher rates, more wars, less rpg
All three servers are good.
EDIT: Worth mentioning is that i also get the feeling that Tibianic & Realots are more serious and will last longer.