For shorter trips I always bring frsh coffee (usually Peet's) and brew it with a filter. However, for longer trips I bring instant and to help with the so-so flavor, I put in a half packet of hot chocolate (normally I never sweeten coffee)
However, I'm heading out on the JMT this summer and I have hit upon a new idea for flavoring the coffee (don't laugh!): I'm going to dissolve a spoonful of Nutella in it: chocolate, sugar, protein, and a dose of fat. I tried it and it isn't bad. A nice little calorie kick. I'm also switching from Starbucks instant to Megdalia d'Oro, an instant expresso. I can't say for sure that it tastes better than Starbucks, but it's pretty good and a lot cheaper.
By the way, my brother, in his "normal" daily life , likes to dissolve a Milky Way bar in his coffee, and then add non-dairy creamer. Ugh! But it might taste good to me out on the trail if I had it:)