The best tasting jello is the simplist. Boil a cup of water, add it to the powder, wisk thoroughly, but substitute a cup of cold Coke for the cold water. The only trick is to trickle it down the side of the bowl since it will foam like mad when it hits the hot water. Now explore the possiblilties--Lemon Jello and Sprite; strawberry jello and strawberry soda; Black cherry jello and coke. Coke and Sprite/Seven-Up will mix well with any flavor, but try to mix and match the fruit flavored sodas with the same fruit flavored jello.
Have fun with all the possible combinations.