Probably my cell phone would be the biggest. I seriously have like a mini panic attack when I lose it for awhile, and just feel disconnected with the world.
I think that my laptop takes the first place and cell phone is the second one! TV is on the last place....believe me or not, I don't need it!!!
I can't live without the internet.. The feeling of waking up, getting up and going straight to a cold keyboard to check your mail and news is a must to start my day.
Oxygen, Breathing.. they are boring answers obviously one can not live without it.. So you've asked about THINGS? So my answer is, I can't Live without by Computer & Internet