Guild Wars 2

So I want to make an alt for and I’m wondering which class is better overall in PvE and WvW?

Mesmer .

Mesmer. They are good in WvW and PvE but they are bad at low levels. Like thieves they shine after level 40 with the Tier 2 Traits.
When you do get level 80 I suggest superior Runes of the Centaur so you can have a constant speed buff using Ether Feast and Curtain.

Mesmer as well. More complex but more rewarding.

Thief is straight forward for the most part.