My favorite is Crank Energy. It's cheaper and better than the alternatives I've tried.

I like the 150 calorie packets they use better than the 100's that most other companies use, their gels include electrolytes but not caffeine, and I think their products taste better than the Hammer, Cliff, and Gu offerings. Also, Crank gels are thinner so they go down a little easier. Disclaimer: if you click on the link above and order from them, I get a discount the next time I buy. But that discount wouldn't interest me in the least unless I was going to buy anyway.