My knife and zip ties. Can use the knife as a makeshift screwdriver, hammer etc. for hardware, section out busted hoses if needed. Zip ties manage to hold many things in pace if needed ( at least long enough to limp on to someplace better to working bike).
:)

Keep a Leatherman in the tank bag, do not recall ever using it. - Go thru a lot of cheap, POS, Chinese lock blade knives. - break em, lose em, get p'd & toss em
Small Crescent & a collection of bits in a plastic holder get used a lot.
The really "go-to-hell" "tool" is a shank of really strong line. - Amazing what you can lash together when all else fails, not to mention making a field expedient "winch" when stuck up to your axles, not that that ever happened on posted land or anything.

I gonna have to with percocet on this one lol

I keep a tool roll on the front end of my bike & have 2 chrome teardrop tool boxes (one on each side of my bike. I have a rolling shop! When you ride antique bikes, you learn to carry lots of stuff if you take long road trips. I have bailed out many a broken down biker on the side of the road. I have extra spark plugs, extra inner throttle & clutch cable with end menders, a CO2 tire inflator, tire plug kit, a roll of #10 & #16 wire, wire nuts & heat shrink tubing, fuses of different sizes and Amperage ratings, electrical tape, duct tape, baling wire, hose clamps, a few different nuts & bolts, extra bulbs, master ling for 530 chain, tube of silicone. Tools are combination philips / flat screwdriver, 3/8" socket set with 1/4" - 3/4" sockets, allen wrenches SAE & metric, combination speed wrenches 3/8" - 3/4" large channel locks & crescent wrench, chain breaker, spark plug wrench, flashlight, wire cutters, small volt meter, snap ring pliers. I have broken down many times over the years & never have I had to call out a trailer! To me, the breakdowns & repairs on the side ofthe road have been some of the most memorable experiences and also make for good stories, like the time my header pipe broke off on the Indian Nation Turnpike and used an old coke can, baling wire and pipe clamps to get me home. One tool? I couldn't pick just one.

WELL I WOULDN'T BE OUT RIDING WITH NO TOOLS THAT'S ASKING FOR HARDSHIP.. ALWAYS BE PREPARED I HAVE TOOLS AND DUCT TAPE AND ZIP TIES IN MY TRUNK, BUT THE DUCT TAPE AND ZIP TIES ALWAYS END UP IN MY BEDROOM.........LOL

I always carry a full tool set too. But if I had to just choose one tool and one repair item - it would be an adjustable wrench and duct tape.