I like hibiscus teas, they have a mellow floral and subtle sweetness to them. Red Zinger by Celestial Seasonings is really good. You can also find plain hibiscus leaves in most spice stores. I have just discovered coconut nectar as a sweetener. See if you can find it in your grocery store/health food store, or ask if they can order it. It's mild tasting but adds a good amount of sweetness. Stevia is so bitter to me.
Also, be careful of the fruit flavored teas. Check for sugar. They use fruit juices to sweeten things sometimes.