Used crappy irons for a few years. When I got into wrenching on real cars and doing electrical work 25 years ago, I got a Weller WLC-100 and thought it was a great upgrade from the $15 specials. It worked, but took forever to heat up and was easily exhausted. Fast forward 20 years and I finally got sick of it, and bought a Hakko FX-888D... I'm pissed I spent 20 years with the Weller and didn't go Hakko earlier. Heats up in seconds and solders anything I can throw at it (I've only tried up to 10ga though).

I also got the Arrowmax soldering iron kit for mobile pit use, and it is great. I did the 6S battery thing to run it at 24v for a while, but after forgetting it too many times, I tried it on 12v directly off my power supply, and it still solders surprisingly well - I have no problem soldering the most expensive motors with 12ga wiring with it. In and out in seconds, just like my FX-888D. I don't even bother with >12v anymore.

The two biggest lessons I've learned in 25 years of soldering:
1. When soldering heavy gauge wires, use the biggest tip you can fit in the workspace - it acts as a thermal reserve and keeps the tip hotter, longer during the soldering process.
2. Use a clean tip. In the pit, I use at least a wet sponge. At home, both wet sponge and brass wool at the ready.