It really depends what you want from you full-back, and how reliant you want to be on their attacking prowess.
To me, the best right-back in the league is Zabaleta. Perhaps the only thing missing (and I'm being picky) is a whipped cross and dead ball capabilities. The rest he has in abundance. Energy, Courage, Goals and determination.
The best left-back I believe in a defensive sense is Cole, closely followed by Clichy.
In an attacking sense it's Baines closely followed by Kolarov.
In short the ideal left-back (forgetting Bale, who no longer plays there) doesn't really exist. What Kolarov does give us, that must be replaced if he goes, is his free kick ability. When we get a freekick with Kolarov on the field, I always feel we have a chance of a goal. Sadly if he is not on, the free kick rarely bears fruit.
If I had to have just one of the full-backs mentioned then it would be tough to choose, but for me I might be tempted to get Baines. In short, goals win games and we need to ensure that any player in our outfield team can score at least 2-3 a season minimum.