Re: The strongest admiral
We haven't seen enough from any of them yet to have a clear winner. We surely haven't seen the full extent of their ability to use their DF in battle, nonetheless special skills, haki, stamina, etc. I mean, all three of them are logia, so it'd pretty much be them throwing attacks at each other until one of them couldn't become their element anymore.
We know that Ice (or water in solid form), light, and lava (fire in liquid form) are "the elements that destroy", in shonen all of these are destructive forces of nature, and all of the admirals have shown great feats with their devil fruit.
I'd immedietly knock aokiji out of this. His ranged attacks seem fairly limited. He'd excel close range, except that kizaru is faster than him and akainu surely could become far too hot for aokiji to handle.
Kizaru dropped four supernova captains by playing around with them (albeit there was some unecessisary help from a pacifista). We've seen him able to have large scale attacks (when he decimated the mangrove), fight melee, and fight with ranged attacks. He also seems to have more experience than the other two, but this isn't for certain. And as many others have said he has speed on his side.
We've seen but one move from akainu. He was the first one to destroy a pirate ship in Whitebeard's fleet, and he evaporated an iceberg that was larger than giants. HIs attack also caused a "rain of fire" that may or may not have affected allies.