My favorite pair would be fortress & Tower cause if i can get styg from fortress with all magic classes (fire earth water air) and get implosion from tower with master genis and flies from the fortress with there weakness effect i lower the stats of opponent and cast defensive spell on myself.
i reckon that castle and rampart would make a pair....not for any certain reason, i just think that
or maybe castle and necropolis because if the regeneration factor (archangels/vampire lords) and good group shooters (marksmen get 2 shots and power liches shoot a group) and then the archangels could resurrect anyone that dies