My three extremely-hard-to-acquire forwards would be Kessel, JVR, and Kadri, with JVR joining the list this year. He's been more consistent, used his speed more effectively, and has shown good creativity -- last year he seemed to be more restricted to dirty goals around the net.
Lupul I still like, in particular because he is one of the guys who seems to be able to generate offense regardless of who he's paired with. The problem is that he doesn't do much for you when he's not producing. He doesn't do too much defensively - doesn't PK, and not on the ice protecting leads, for example - and doesn't play an overly physical game. He's strictly a top six guy, and as he gets older, if his production drops to the point that he isn't worth putting on the top two lines, he's not much good to you.
Definitely a guy I'd be sad to see go, but if the goal is to build a cup contender, I'd be looking to trade him in the next year or two for a younger guy who won't be declining just as the core (hopefully) has matured enough to put together a good playoff run.
Of course, if you trade him, there goes our winger depth - at that point, we're just a team with two good wingers that make up for our weakness up the middle, and some decent 2nd/3rd liners. So the return would need to be good.