Only one?? Yikes.... I could never bring just one... Lol, just couldn't do it...
I suppose if I "had" to choose only one... It'd be either my ESEE-5 with Hedgehog Leatherworks deluxe sheath.... Or my Dogwood Custom Knives Echo-5... It'd be really hard to choose one of those two.

But in all honesty, I'm very into bushcraft & survival so I go quite gear heavy and always carry a knife for each individual task, as well as folding saw & hatchet.... so one just wouldn't be an option... It'd be both above, as well as a Bark River Necker 2, Survive Knives Necker, Gerber Suspension and Benchmade Rescue hook. Ya, I know... Quite a lot, but it works for me and I like it... Lol. And surprisingly it doesn't add much weight at all (though the ESEE alone is a solid pound, lol), nor take up much space...

(The ESEE/HH, Bark River and Survive are in my profile pic. The ESEE 5 is one mean knife... Pretty hard to beat)