I technically have 3 mains, cause I have a problem ;w;
But I guess my first/oldest character I chose Thief cause I had a love for assassin-type classes at the time, and as a Sylvari cause I LOVED the idea of a plant-like race instead of the traditional fantasy elves (and now I have 12 sylvari).
I rotate between this thief, now a Deadeye (I LOVE the whole sniper aesthetic), my Weaver (I just love how Weaver plays, it's so fun and fast paced) and my Mirage (basically a clone of my FFXIV main) because I always find stuff to do on those characters specifically.
I mostly PVP/do openworld/WvW in here, I'm not sure which character I'd pick for RP, of if I'd even do it in GW2 at all...
I have switched mains many times over the years, but I tend to do pros and cons list of the classes and see what I really want. Right now, for the first time ever, I'm trying to stick with mesmer. Although my longest mains (in order) were: Engineer -> Elementalist -> Thief -> Warrior -> Revenant -> Mes (only a few days so far, so we'll see. haha)
I mostly hop around between Ele, Rev, Thief these days. But I've been looking for something fun in open world, flexible with good support options, good mobility, and enjoyable in PvP/WvW roaming too. I narrowed it down to Ele or Mes, and since I never really got into Mes, and with so many story chapters to do now, I was hoping to try it out and see if it would stick. Otherwise, I'll be back on the Ele, Rev or Thief.
I just find Thief gets boring in PvE (open world), and Rev gets repetitive after a while with limited build customization. Which is why ele pulled ahead with mesmer, since they are more flexible and less repetitive in open world.
These days I play:
60% Open World/Story
10% WvW Roaming