What is the most valuable tool you own?

My brain.

Grew up with a mechanic father, mechanical engineering tech degree, and lots of knuckles busted have me thinking about the repair before even lifting a tool.

Monetarily, a really nice finger brake.

Spiritually - I bought a $15 Wel-Bilt angle grinder in 2014, in college, when I bought my first welder (with partial loan from a friend lol). It is still running perfectly, extremely smooth, and is likely my single most used tool ever.

Google and my 3lb shorty hammer.

Valuable to me? My 1947 South Bend Lathe. 

Valuable in monetary terms? Probably still the lathe. 

Past berkeleyups and failures.

Sentimental? My grandfather's masonry tools.
Money? The 4Runner. It tows, carries things, gets parts, goes anywhere!
Use? My micro tooth 1/2 ratchet. 

Almost always the factory service manual for what I'm working on, with various forums coming in close second. 

After that, the window AC if it's summer here in FL...

I still have a 1st edition of this in the garage and I still use it. I have had it for almost 27 years and it is the single most important book that I have ever read for how it effected my life. I remember my Dad and I buying it used at a swap meet. 

Got me though keeping my first car running. 

Got me going when I started buying VWs and fixing them. 

Is the reason I am so good with my hands. 

My snap-on ratchet. Sometimes I'll go out to the garage and rotate it to hear the teeth engage

My DVOM. 15 years ago I decided to specialize in electrical diagnosis for the thrill of the hunt. Now I'm really good at finding the problems other people don't even understand in miles of wiring. Power Probe 3 is a close second.