Time for the classic deserted island question, but modified a little. You, alone, plenty of supplies for life but no entertainment except for one geek hobby of your choice. And yes, you have access to not just a single item, but the entire hobby.
Currently I'm thinking video games for myself.
So what geek hobby would YOU retain access to if given the option to only keep one?
I think I would have to agree with video games. Given you had all the necessities of life......man I can just imagine myself in a cozy chair w/ a big pizza and a case of pop playing games for like 3 days straight! Crap, now I want to put in for vacation the rest of the week!!!
Now I guess that your length of stay and availability to new games as they come out would play in to the answer........I mean were it only one game for the next year, I may reconsider.
Either way, a deserted island (ie.....peace and quite!!!) sure does sound nice right now!
If you really mean geek hobby as in all kinds of geekiness, I'd say "all things gadget." I really LOVE my gadgets, using them, reading about them, even blogging about some of them. So my gadget hobby would go with me to the island. Access to the internet and all of my apps are implied of course, since every smart gadget I use is "connected" and doesn't work without its apps. And I'm counting my iMac as one of my most fun gadgets, by the way. (guess I'd be a one woman tech centre on that island, watching the surrounding flora & fauna via my Mac screen or through the lens of my iPhone's camera... )
If it must be geek entertainment, then I'd say comics.
I'm going with video games galore, or access to every single science fiction novel ever. Good or bad, at least it would be there to read. And as Rickster pointed out, playing with a tablet would accomplish both goals, so I might have to go that route.
Oooh, I'll play. I'd want my knitting stuff. That way I could both entertain myself and make life on the deserted island more comfortable at the same time. Not only could I make clothes for myself and furnishings for whatever shelter I could manage to construct (maybe I'd just loom a giant tent over a frame of branches), but plenty of yarn and large pointy aluminum needles might also prove useful for trapping and skewering whatever edible creatures the island provides.
I'd have to go with hand loading ammo and long distance shooting. Not what people normally think of with a geeky hobby, but I like shooting and trapped on a deserted island with a nice enough range would be nearly perfect.
Plus, if anyone tried to make me leave, they could regret that decision.