Right now, as of 0.6 and 0.7, AoR (Renewal) and AoD (Defense) are most popular. But of course, you should always choose something attuned to your playing style and to your liking. Most of the guardians in my guild are AoR build, and should u go this route, putting most of your attribute points into stamina is a must and +AA gear also to help w/ other attribute requirements (AoR works better the more health you have). AoD allows you to take more damage (obvious) with high mitigation. However, AoD is going to be re-balanced in 1.0 patch and will give less defense. I went with AoD and love it, but ultimately it's your choice.