To my taste I find that a little sugar enhances the flavor of most teas, much the way salt enhances the flavors of food. I don't add it to really good green teas, but I might to their second or third infusions if they're starting to get a bit tannic. But certainly Chinese oolongs and black/reds seem to be enhanced by a little sugar.
I only add milk to really strong Indian black tea, like Assam or Nilgiri or blends in which they are the main component.
I never add lemon to any tea, unless it's crappy iced tea (made with Lipton, say).
Regards, Dean