Paul,
as Mr 'Naut says, it's a matter of finding one that you like the taste of and that doesn't cause you to either projectile vomit or cause the world to fall out of your bottom. Ask 5 different runners which one is best and you'll probably get 6 different answers.
Most of the liquid gels are a mix of maltodextrin and fructose, some are blended to be isotonic for more rapid digestion (SIS and High-5) and don't require extra water- the downside to these is that they are bulkier and offer less calories than more concentrated ones. You'll also find gels with added caffeine or l-glutamine.
My personal preference is for the SIS Orange gels as they have a fairly bland taste.
Edited: 02/08/2012 at 15:10