Both in their prime.Neither can use their armies.Which means Arthas cannot summon undead and Lei Shen cannot summon Mogu.
The Lich King at his peak i.e Wrath of the Lich King.
The Thunder King at his peak i.e 12000 years ago when he was Emperor of Pandaria.(NOT THE LEI SHEN FROM MOP)
(I,personally,go with Lei Shen.)