Which one of the green lines is the best one?

It does not really matter!

All the suggested moves are excellent. Inferior moves are never suggested, this is why sometimes you see less than 4 suggestions. Just pick the one that looks the most natural to you, and that fits your playing style.

If you really want to know:
  • Thicker = slightly better
  • More opaque = slightly better

So thickness and opacity are saying the same thing.

But, again, you don't need to focus too much on this. All the suggested moves will win the game. Picking one is a matter of playing style.