I'm a fan of the Hammer gels. I think they are tastier than others I've tried and they go down easy for me.

When I ran my first (and only) marathon, I didn't do gels at all, but took the sports drink on course every 2 miles. I trained ahead of time with the type they would have on the course so I would be used to it. I found it very convenient not to have to worry about carrying gels or anything. At first, I found the drink (Cytomax) way too sweet, but once I got used to it, I was fine. And now, I actually like it!