Tool - Topeak Torq Stick - 4-20Nm
If it's your first time putting a bike together and you're unsure about how tight components are supposed to be, it's reassuring to use this tool and match the torque to exactly what the manufacturer specifies.
We have a couple torque wrenches around the shop of varying brands, but this Topeak "Torq Stick" is the staff favorite; each mechanic requested one of their own.
It's fun to use - set your torque, pop one of the many included bits in the magnetic cavity, and tighten until it clicks. Voila! I (Will) used this tool on my last bike build and was slightly surprised to learn that while I never undertighten, I frequently overtighten, specifically handlebars, front derailer clamps, and pinch bolts.
It comes with 9 bits, and you'll likely use the 3, 4, 5, and 6mm most often, although I did find the t25 Torx bit handy for the threadless stem on my tandem. Mark gave me some alloy chainring bolts the other day which I usually avoid because I'm apparently heavy handed and have a tendency to strip them out, but I was able to install them easily with the Torq stick.
If you're messing around with carbon components or frames, please use a torque wrench.
Make sure you always set it back to 4NM to keep it calibrated as long as possible.
You can probably find faceless online retailers selling these for below MAP (minimum advertised price), but if you're able, you should buy it from the people who introduced it to you.