What is that one tool you carry with you to every service call that you just cant live with out???
other than the standard, long 5/16 and, electrical meeter, and yada yada,
I'm looking for the kind of tool that you did not know you needed until you had it.
Hmmmm. Well, I can't really think of any ONE particular tool that I could not do without on EVERY service call ... except, as one previous responder mentioned ... my own brain and it's knowledge and experience.
One of the problems with your question is that many of those who might give an answer to it work in various different areas of the HVAC field. Therefore, any particular "must have" tool might vary between them depending on the specific kind of work they're doing (type of service call they're making).
For instance, right now I do almost exclusively DDC controls. In previous employment I did almost exclusively boilers in yet another job ... almost exclusively package AC units, in yet another job ... almost exclusively large centrifugal units. Etc.
In general, I'm sort of the lazy type. I tend to hate packing around a bunch of stuff I may or may not need. And tend to want to see what I'm up against as concerns the trouble call before I start breaking out a lot of stuff from the truck.
So, over time, after having worked with a lot of different equipment for many years, I pretty much settled on a standard "load-out" of stuff I carry pretty much all the time. It's small in number, weight and total size. And as I'm given to wearing clothing with sizable pockets (including "cargo pants"), it all fits easily into my pockets or pouch on the belt.
Mag-Lite, 2 AA battery type, with LED bulbs
Multi-tool, Buck type (just my own preference)
Folding Hunter style knife, Buck ... again
Couple "tweaker" screwdrivers ... small ones in shirt pocket
Klein 10-in-1 screwdriver
Klein Wire stripper
Roll of electricians tape ... used for all sorts of purposes
Couple home made jumper wires ... alligator clips plus On-Off switch
Pocket DMM ... usually Fluke T5-600 or equivalent
Small parts case ... with a few wire nuts, etc. Fits in any pocket
Pocket digital thermometer
Between all that stuff I'm often able to fix or at least do preliminary diagnosis of equipment to narrow down to where the source of the problem lies.
My first objective is always to figure out WHERE and WHAT of the problem. Or, to establish "what it is not". From there I can decide more accurately what other tools and/or materials I need.
Did I mention that I HATE dragging around a lot of stuff around that I end up not needing? :-)
With the items I listed, I can usually get a pretty durned fair idea of precisely what I will need to fix the problem or do a further, final diagnosis.
As far as your question as concerns the kind of tool that I didn't know I needed til I had it ... not a lot comes to mind. Except, perhaps two items.
One is a set (metric and American standard) of those combo wrenches with the ratcheting box ends. Geez, they can come in really handy at times.