It depends on what you prefer as a "carrier." Stevia straight is very, very, very, very sweet, so it must be diliuted in something. For liquid, it's usually either alcohol or glycerin. For the powder, it's some filler (can't recall). I've found it's easier to get a small enough amount with the liquid than with some of the powders.
For addiing to tea or water, I prefer a liquid.
I also use a powder called "Stevia Plus" (by Sweetleaf) that is diluted in inulin fiber.