Here's to the (in)famous Spider exercise or, as you can also call, the 1234 exercise. It is the only exercise I've never stopped playing since I've started playing the guitar. By this time I can still see why I and my students can really benefit from it. It is one of the most complete practices there are for the guitar, it helps to improve both your hands and posture as well as the way you stay on the tempo if played with the metronome of course.
The idea is simple: you play the first 4 frets of each string starting from the top 6th string and finishing with the 1st. I always suggest my student start with a slow speed with the metronome, like 40/50 bpm, by playing each note with a click and, as they improve, speed it up 5 bpm at a time. Let's not forget the coming back though, the 4321. It is basically the same thing but backwards.
It is important to make sure you keep the right posture with both your hands and that you alternate the picking with the right hand, down and upstrokes always.
This goes without saying that it works for all kinds of guitars!
Here you can see both TABs for the 1234/4321 exercise: