Ace can use Busoshoku Haki as confirmed by the canon novel One Piece Novel A, so he'd be out of the question for Enel beating. In terms of power, Enel's strongest attacks could probably rank as stronger than Yonko 2nd in Commands or maybe even Admirals based on his feat of destroying an island like Birka and being implied to be capable of destroying Skypiea, but even New World fodder could speedblitz and oneshot him before he got a chance to do so when nearly everyone with a name has Haki. Strongest non-Haki user that comes to mind immediately is Current Franky, maybe one of Doflamingo's executives like Dellinger (don't remember him using Haki). It's hard to remember who isn't a Busoshoku user in the current arcs when it's become something of an unwritten rule that anyone worth their salt in the New World can do so.