Wait. What?? I'm not sure I understand this question. "Only one"? I'm so confused!
OK - I'll play along. If you had asked me this question 10 years ago, I'd've said an acoustic of some sort, probably my D35 since I've got more history with that one. 5 years ago, I would have probably answered a specific Strat that Mrs Snarf bought me about 10 years ago now. These days, I'd probably say my Epi 339. The size and weight on it are really comfortable, and it's pretty versatile. It can clean up and get those airy, semi-hollow tones I've decided I really like, or it can get mean and nasty like a solid body dual 'bucker guitar. But that's also assuming that I have an amp, so I'm half back to saying an acoustic.
If I get an amp to go with it, I'd have to go with the 339 and my 7/15 watt Blackheart amp.