No armies? Then Lei Shen wins, the Lich King was mostly about armies. Heck, even in his fights we get val'kirs trying to throw us around, angry spirits in the frostmourne dimension, etc. Sure, Arthas defeated Illidan, but Lei Shen was several times his size and made of rock, so frost attacks wouldn't do much. Furthermore, Lei Shen can summon the lightning while Arthas has a metal helm on his head.
With armies, the Lich King would win most likely but 1v1 Lei Shen takes the cake.