Only one? Hmm,, I'm torn between a general purpose built knife or a Multi-Tool. If we're just talking knives, because the knife blade on most multi tools is a convenience more than anything else, I'd go with a Fixed Blade with a full tang. The Gerber LMF 2 I keep in my BOB has been a decent knife. Good 420HC Steel, it takes and keeps an edge. Only a 5″ blade, so it's not a monster, but a good general use length. Comes with a sheath that can be worn on a belt or around your lower leg. I suppose you could mount it on a shoulder strap for quick access. Around $100 – $120 to buy, it's not a terribly high priced knife compared to some other manufacturers.
Being retired on a fixed pension, Benchmade are a pipe dream for this old fart.