I've been trying the LorAnn "Flavor Fountain" flavors in 2.0 (1 teaspoon/bottle).
They're thick, almost like a gell, so take alot of agitation to blend in, and also seem to get stronger if you let it sit for a bit. If you just dump it in, shake a little, and drink immediately, you won't notice any flavor at all. I've tried lime and orange so far, and have other fruit flavors to experiment with.
eta: also make sure to shake the flavors well before measuring - the gell part tends to settle and that's what carries the flavor.
also, the LorAnn flavors are also strong colors. I like it, but check for mold first, before adding the flavor.