re: Guitar guys??? BEST training exercise?

I'm a professional double bassist. It is a lot, but if you're looking to fill your time that's a start.

The guys at the top have a ton more talent than me and many still practice hours a day. If you've got the time it's the right way to get better.

Eta: Just saw your goals post. I'd still run a lot of scales for finger independence, but I'd focus on playing the most common chord progressions and playing them with good time. Play with a drummer, drum loop or metronome if you can.