Wow. Now, most of these fighters would have been able to go through in another group, but since this is a Royale...
First of all, as strong as I'm sure he is, I'm not going to vote for Gol D. Roger. He was the former pirate king, but since we've seen nothing from him I don't like voting for him. Otherwise it's just gunna be a Shanks vs Roger final, and that's no fun. So while I'm sure he's strongest, I'm not voting for him.
Now, Vista and Mihawk were fighting pretty evenly, but Mihawk is the greatest swordsman in the world, and has shown to be obscenely powerful. Vista is the captain of the 5th division, and one of the best swordsmen in the world, but Mihawk is in a league of his own. So by that, I'd say Mihawk > Vista. Pekoms is massively strong, but he's the weakest fighter here IMHO. Mihawk, Shiki, and Vista are all obscenely powerful swordsmen - Vista is a swordsman who's close to Jozu in terms of power, Mihawk is probably the strongest Shikibukai and the strongest swordsman in the world. Now, Shiki could fight Gol D. Rogers, and survive - he was a hugely powerful swordsman, and had a fruit that was almost broken - throwing islands and the like.
Since I'm not voting for Rogers out of principle, then I'm gunna say Shiki and Mihawk, but were I including Rogers I'd say him and Mihawk - Mihawk's almost certainly the better swordsman, and showed vs Don Kreig that he can cut large objects. Now, if Zoro can cut through a tsunami, then I see no problem in Mihawk cutting through that much water, or even land, so he can potentially get the upper hand...