In my opinion it's cesc. Aresnal can manage without henry (no one man team) but VP will surely be missed. VP is the flying Dutchman and I'd love to have him at spurs.
Valuable in what way? Price? Goals? Team work? If price it's Henry obviously as he's the best striker in the world. If to the Arsenal team, I'd pick out of Henry and Cesc. Cesc more at the moment as you can cope without Henry as shown over the last couple of weeks.
I would say cesc is the most valuable because he is the youngest and is a big player in the midfield for Arsenal
In terms of money? Henry, but he's only got a few more years left at the top. Cesc would be a big investment for the future for any team willing to splash the cash on him. And as we know RVP is pure class.