Phoenix, I also try to be sugar and caffeine-free. Have you tried Fresca? I think it's much better than Diet Sprite. I also like Caffeine-free Diet Dr. Pepper when I can find it.
I haven't heard of urge. I've heard of Moxie and would love to try it, but haven't had access.
Best root beer: IBC
Best ginger ale: Vernor's
There's a product called Jolt Cola I haven't seen for awhile, that boasts double the caffeine and double the sugar. I don't care for the sugar, but if there was a diet Jolt with double caffeine I would be in sodapop ecstasy. I hope they aren't out of business.