I used to have a few favs, but as time has gone on and I've brewed the same styles and/or tried more commercial brews I don't really have a favorite. I can never turn down an Imperial Stout or Doppelbock though.
I wouldn't file it under "can't live without", but if I had to drink just one beer until the end of my life, I'd pick Augustiner Helles. While I love my hoppy bitter IPAs and crisp pilseners, Augustiner is a basic no frills Lager done perfectly.
Don't listen to me, for all you know I'm just some random drunk with internet access.
I truly enjoy almost all beer styles. If I lost any one of them the others could easily take its spot. My two favorites are big Belgiums and very hoppy double IPAs. Lose one wouldnt be a big deal, but lose both I would be quite disheartened.