Find the dark agave nectar. It has the most flavor It's usually listed as "raw" and you're more likely to find it in the organic section of the store or maybe with the honey.
There's a honey-colored nectar that will do ok but it won't contribute a lot of flavor. Then there is agave syrup which is completely refined. You might as well just add table sugar.
Forgot to mention, it tasted better the day after it was opened. Somehow the agave seemed to become more pronounced and the beer became smoother. Just give it a try, but make sure to pay attention to the amount of sugar in the nectar.