The only decent coffee I've had on board a plane was an espresso on SQ: they seem to have decent ground coffee and proper espresso machines on board.

Otherwise, I always go for tea: there's only so far wrong you can go with tea bags (well, unless I see 'Lipton' or 'Pickwick' or similar). Coffee inevitably ends up being a horrible, sour, light roast, which I can't stand: coffee should be dark and bitter (IMO).