Well, as you can tell from the replies "best" is a very subjective word.
campingcorgi seem to have covered it very well.
With that said (and even if most folks don't like it) I think the 1900 loop is best. Nice, dark and quiet!
No matter which loop you get, you will have a great time! If you plan on returning in the future, take a walk around all the pet loops and make a list of sites you like. Next time, make a request for specific loop and/or site. I have a list of sites I like and just hand it to the CM at reception. They are always happy to see if any of the sites are available.