I'm going to go a bit differently, and say my machinist tools. I keep a 6" Starrett hook rule and a 4" combination square in my front pocket, and use them constantly. My Kennedy box full of various Starrett tools sits right on my knife bench, and everything gets used.
If I was to pick a few specifics out, I'd say a 6" hook rule, a diemakers square, 4,6, and 12" combination squares, a couple of the offbrand machinist squares, a good scribe (I like the Starrett style myself), a good 0-1" micrometer (for me calipers are basically a direct reading hermaphrodite, micrometers are for measuring.) I've got probably 5 different 0-1 mics, and mics all the way up to 8", but one good 0-1 is all the knifemaker needs. I'd also included a decent surface plate, a surface gauge, and a height gauge. I've got a few height gauges, but the 4" mitutoyo dial is used the majority of the time.
I've found the more time and care is spent on good layout work, the less time is spent correcting mistakes, and wishing something fit or looked better.
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Blue painter's tape. How did anyone survive without this?
they used masking tape insted 
As for the tool i could not live without... well aside from my forge i would say its at the moment my bench grinder for now as i use it when strating my edge so i do not have any waves in the edge (unless there purposefully put there) and then to clean up my tang when i plan on attaching a guard to my blade.
PS: actually forget what i put, during this time of the year the one thing I could not live without is the fan i have in my little garage forge. Despite living in Minnesota were we are known for are cold winters, the summers are even worse, with all the lakes, rivers and streams around us it gets humid enough that it rusts my scrap metal. god it got to 93 last saterday and i actually had to take off my shirt to work. actually also my drill doctor as i am not the best when it comes to drilling holes and this helps me a great deal. then again it helps when i remember to drill my hole before i heat treat my steel.
