I always have two, often three.
The two:
Wenger Esquire(2.3" closed, swiss army keychain style, has blade, fingrnail file/cleaner, scissors, tweezers, & toothpick- small & innocuous enough that it offends no one, amazingly capable)
Kershaw OSO Sweet(substitutes: Kershaw Vapor, Benchmade Mini/reg Griptillian, Benchmade Osbourne 943- those are the main ones)
The third? Might be almost anything- mostly regular size pocketknives like Opinel #6-#9, old Case 3-blade Stockman(3347 pattern), Victorinox Swiss Army (mebbe Soldier, Farmer, Climber, Explorer, or one of many others); bigger to much larger, such as Benchmade McHenry & Williams #710, a big no-name locker often carried during yard work, Kershaw Vapor II, CRKT Carson M21-04, or others.
BTW- dgee, of all the foldng single-bladed one handed opening, locking knives I own-- and there's a *Bunch* of 'em- my favorite of them all, by far, is the Benchmade Osbourne-designed 943.