Smoker not only dealt significant damage to Vergo in their 2nd battle, but was also trying to get Law's heart back, so his motive wasn't purely centred on killing Vergo. As for Law, he was heavily injured by the time Luffy arrived to fight Doflamingo, so I wouldn't say his feats in that battle are the best way to place him. Moreover, his feats on Punk Hazard would seem to suggest he's >> Gear 2nd Luffy based on the previous chain, as well as Luffy needing Gear 2nd just to take down Smoker in Tashigi's body.