Mine is a 8,9,10mm three-sided wrench. Found it in an old handlebar bag I bought off CL years ago. Probably gets more use than any other tool, excusing the hex wrench set.

My favorite tool is something I just put together myself, about a year and a half ago.
My headset fork crown race setter.






Just a trip to the hardware store for some PVC plumbing items (About 6 bucks worth) and some thick silicone rings (To center the steerer tube in th PVC tube) I salvaged from empty prescription bottles a few years ago.
Works a treat and is now a veteran of five headset race installs it is a surprisingly effective tool that just cost me a few bucks.
The bonus was my Park headset race remover tool fits inside it for storage, so all the headset tools I need are in one place, except for the lock ring wrenches....

+1. Working in a bike shop in the mid-1970's a wholesaler's sales rep was showing the owner of the shop one of these, and I suggested he buy me one -- and, if he didn't see me using it every day, I'd pay him back out of my own pocket. Never had to pay up. That said, I bought one for home use, still have it, looks like the photo.

Axle vise
Let me know if the pic shows up.

1. Oxy-acetylene torc
2 . Big vise.

Since this is the Classic and Vintage Forum, my favorite tool is my Cinelli Work Stand that I purchased 33 years ago and still use it to this day. Even though I bought the Feedback Ultimate and Sprint stands, I keep going back to the Cinelli because it just is sturdier and sure footed than the Feedbacks. Some of it is starting to fall apart and disintegrate but it still works good enough. It doesn't have the bells and whistles of the other stands but it has stood the test of time.

Sugino BB tool, never fails, never damaged any cups or frames.

A good cone wrench makes me happy.

^ What a great idea merziac. I was using one the other day and yes it popped off a few times.

Tx, the cheater wrench is also key when stubborn and the piece of wood has a hole in it now so you can use a big socket as a spacer if the axle is still in place.

+1,

I like those 8-9-10mm Y wrenches too. Also indispensable is the corollary 4-5-6mm Park Y allen wrench tool.

My personal favorite tool when I was wrenching was the Hozan 4th hand tool. Still have one.

No contest. The Campy "T" wrench was the original "multitool". Not so many tools, and it could have been better designed with perhaps two different hex keys, but a lightweight tool that met many needs and was elegant to boot.

Third hand tool for brakes. Not really bike specific, but a little 1/4" non-ratcheting driver with a 90 degree bend in the shank. Great for brakes and whatnot where having to switch ratchet directions can be annoying or difficult. Hard to overtorque with.

I like it.

I recently got a Pedro's Vise Whip in lieu of a traditional chain whip, and I love it. It's like a chain whip and vise grips combined, and it makes cassette/freewheel removal quite a bit easier.

Tx, works like a charm.

Interesting. Chain whips were originally intended for taking freewheels apart, cog by cog, not taking off cassettes. They got repurposed because every shop had them already. 30+ years later, it makes sense that there's a purpose designed tool for it.

There were at one time chain whip alternatives for freewheels that were somewhat similar to the Pedro's tool. They didn't have the strength of a simple chain whip though, and for cog removal, you need all the strength you can get.

I broke a chain for the first time the other day and was able to use a chain tool to remove the bad link and reconnect and keep climbing. That is my favorite tool so far. The one I used was a nice solid one made in England. Started with "R" I forget the name.

Here is my favorite tool from all those years ago, when I used wrenching to help pay for college:

Cyclo Rivoli.