Of the 4 mentioned, Looper will start off with the closer job, so it is his to lose. He's the best bet. If he loses the closer job, it will likely to be lost to either Vladimir Nunez or Tim Spooneybarger. If Looper is available, then both of the others should be available as well. Be ready to pick up either if Looper falters.
Embree is probably the next best reliable. I've seen some reports that have Embree getting the bulk of the Red Sox SVO's... even if they do claim to be going to bullpen by committee. He's not a bad pitcher and he has definite closer potential.
I don't know much about the others, which only means that they're not high profile, not that they aren't good pitchers.