Out of the ones I've used the Go Gels are definitely best. They're isotonic (unlike most of the other ones) meaning that you don't have to take them with water, and they also taste far better (though that is not to say that they taste particularly good, either). The PowerBar lemon & lime gel I tried fairly recently tasted absolutely revolting!
If you get the Go gels, make sure you order the newer ones in the narrower more practical packets rather than the older square ones.
Incidentally, the same applies with energy bars as well. The SIS Go bar is by the far the best one I've tried (far nicer than the High5 ones, for example), and in fact I would actually go so far as to say that it is pretty tasty -- particularly the Cherry & Vanilla flavour.