Wiper is strong, but he's already shown Enel beats him, so he's out. Shilliew is really strong, on Magellan level - definitely stronger than Caribou. He managed to tank a shockwave from Sengoku and get up unscathed. Garp is the clear winner here, though - he used to fight with Roger in his prime, and is deemed by both Whitebeard and Shanks to be very strong, so he's - in my eyes - the clear winner.
So Enel - and probably Shilliew - could beat Wiper, Caribou is essentially fodder vs so many Haki/Logia users, and Garp is the winner. That leaves Enel vs Shilliew to fight. Now Shilliew was in Impel Down, guarding prisoners such as Jinbe and Crocodile, so it's safe to say he has access to Haki, but as we've seen Enel isn't easy to take out - his mantra and speed should allow him to dodge for an indefinite time, plus his offensive power is so great he can do some serious damage. Shilliew is powerful, sure, and he may well end up as one of Zoro's final opponents, but Enel was a lot stronger than his Luffy's defeat of him made him seem, and I think he'd win.
Garp + Enel, with Shilliew a close third