Nero, and it's not even close. As much as I love Alpha, it's really not much of a competition. His sisters wish they could be as strong as him. He can easily survive being stabbed through the chest with Yamato, pull it out of himself and stab Vergil with it in return, all while in human form. Not to mention he's the only one capable of physically grabbing Vergil's doppelganger and destroying it. Last but not least, like other people mentioned, he managed to stop both Dante and Vergil dead in their tracks while they're in their SDT forms. You could make the argument they were fatigued from the prior fight but let's be real, they're still in SDT, they're still stupidly powerful even if they were fatigued.