I think any leader that functions well on levels higher than Monarch, would work great on Monarch too. Most of the names mentioned so far would be candidates for #1 at any level (HC, Liz, Darius, Asoka etc).
One leader I found to be supremely effective at Monarch (although less so at higher levels) is Willem. Simply Rex, claim all the land before the AI has chance to settle, improve the land, sit back, win. If the human has >= rival civ land at Monarch level, they are guaranteed to dominate GNP, Mfg, food harvested, pop etc, and it is easy enough to convert these advantages into the victory path you choose.