I'd say it's not quite possible to know just yet. I feel that the top 3 Warlords Mihawk, Doflamingo and Kuma can battle on par with admirals, but Kizaru IMO is the strongest admiral so far. I will however make an assumption and say Mihawk by the skin of his teeth. This is largely because I feel that Mihawk will be Zoro's final or second to last battle and Kizaru might be the same for Sanji. Given that Zoro's opponent is always a trifle stronger, I'm assuming that Mihawk is a trifle stronger than Kizaru. That's a lot of assumptions but that's about all I can go off of.
It will probably be Sanji s Kizaru (both kick heavy), Zoro vs Fujitora (swordsmen) and Luffy vs the as of yet unseen Ryokugyu (sounds like a melee fighter like Luffy). And Luffy will likely take down Akainu as well.
That would be too much trolling for Oda. Funny as it would be he doesn't troll on that level. Still I INSIST that Zoro beat an admiral soon. I guess it won't happen before Luffy, unless Luffy takes down a Yonko at the same time.