Guild Wars 2

So I have a Necro and a Thief at Level 80 and I can without any doubt say that my favorite is the Thief over every aspect except maybe the support condition damage role in big zergs (which I don’t usually do because I found it boring)
I enjoy the thief in tpvp and spvp, I love roaming in wvw with it and doing map completion really fast having the thief the ability to stealth and move really fast.
I’m always been a fan of stealth and rougue type characters and the thief in GW2 is the class that better fits the assassin role wich I seek in an RPG.
I can’t speak for the other classes as I didn’t try them yet but between the two the Thief surely fits better my playstile even thought in large zergs and dungeons the Necro is easier to play and, I feel, more useful.

For the spvp question I’m currently running a 2/0/0/6/6 s/d build and I find it the more balanced for pvp but I’ve also tried d/p.
For spvp you don’t need a gear because the stats of the gear don’t matter but just the build itself so you can run pvp since level 2 just find the build that better fits your playstile.
For wvw you will need to be level 80 and have at least an exotic gear (very easy to get through pvp reward tracks) I personally took mine in 1 week of pvp doing the Flame Citadel reward track unlockable doing the story mode of the level 70 dungeon.
To decide which class is better for you I suggest trying them all in Pvp and then levelling the one you prefer or even all of them, it’s up to you.