There is really no "best loop". Each loop has sites of varrying sizes, so you need to select your level of service (as Becky pointed out) then make sure your reservation is noted with the length, number of awnings, number of slides and any other thing that will take up space on the site.
The site assigners have a book with each sites length, width and space available for extras. They use that in determining the best site for you. Now, this dosn't always work out, so if you get to the site assigned and it will not fit for some reason just go to the nearest comfort station and call back to the reception outpost. You can usually select from sites that are open.