lvl 80 Booster - Ranger or Mesmer
For solo PvE i will say Ranger.
But i personaly vote for Mesmer cuz more challenging and the unlimited amount of clone/phantasm we can make help us alot more than ranger’s pet
The Ranger is interesting class because it’s elite, Druid, is completely different from base Ranger. To understand how different, you should raise it slowly to level 80 and then see how much Druid changes the class.
Chrono does not change much for Mesmer.
Mesmer really blossoms with many levels, and can be slow/difficult
to level (for some). So I’d boost mesmer, and go through the
normal level-up procedure (including tomes here and there) for
ranger.
Both are fun at max level, and have very discrete playstyles.
I think you should look at which class is gonna have a steeper learning curve. Whichever one you think you can pick up quicker is the one you should insta-level. I’d lean towards using it for ranger, and regular level mesmer. I had an easier time learning and assimilating to the ranger than I did the mesmer. (I’ve leveled both to 80.)