Assuming it was limited to D&D it would be a tough one.3.5 core books are a good place for a complete game in terms of content. A whole set probably SWRPG D6 or SWSE.
I'd have to go for HERO 6th. With it, I'd be able to recreate my HERO superheroic campaigns- some of my best work ever- and even D&D style tuff as well.
I'd grab my metal miniatures, since they're way more valuable than any boardgame. But if it had to be a game, probably rail baron, since it's out of print. If it had to be books, no idea. Those strike me as more easily replaced.
I know it's sort of cheating, but I'd grab exactly three books--my copy of Fantasy Craft, my Rules Cyclopedia, and Savage Worlds deluxe hardcover. With those books, I could likely play any RPG experience or genre I'd ever want to recreate.
Though as soon as other more important issues were squared away, I'd want to replace my copy of The One Ring boxed set. (Even if I never play the game, dang those books are fun to read through, and the artwork is stunning.)
Strangely, everything else I have on my shelf would get a resounding shrug if they needed to be replaced. Granted, my RPG collection is miniscule compared to some I've seen posted on here.
My FASA Star Trek because it just arrived from eBay and I haven't had a chance to read it yet! More realistically probably my 7th Sea stuff because it's out of print, I have DDI, the most recent L5R is in print (and similar to 1st Ed.), and FATE Core is coming out relatively soon.
I hope my wife doesn't stumble onto this thread. If she finds out I can choose one set of gaming books so easily we're going to have a yard sale that I'm not ready to have yet.
I'd probably save my Complete Calvin & Hobbes instead.
If the question was about the book I'd like to have in an island I'd say GURPS 4th edition, Characters due to the possibility of playing in far different settings/ages.