Howdy Kal,
Wow , that's a loaded question for sure because each tool has for the most part a specific job to do.
I think in general, that good tools produce better results and are easier to use ie. you can buy a spru cutter for for 5 bucks at Wall Mart or 11 bucks at a hobby store. I 1st bought the Wall Mart one and quickly replaced it with a good one. It made a big differance in the amount of work to subsequently be done because the good one cut way more cleanly and resuted in a lot less sanding etc.
If I had to directly answer your question, I'd say I can't get along without my airbrush. I fortunately bought the best one the craft store had with a 40% off coupon from Michael's and never looked back. It's a gravity feed Badger Patiot and I'm very happy with it and the results I get using it, but this is not a little tool.
The little tool that is indispensable in my arsenal is a good pair of self locking tweezers I got from Squadron. I use them extensively on every build.
On the low priced side of the spectrum, I find spring closepins to be very handy as clamps and parts holders while painting. I got 2 sizes at a dollar store for cheap.