Which alt should I pick?
I’d go with the guardian or mesmer, unless you are role playing the characters and one has more story for you. For game play and survivability, I’ve loved my guardian and mesmer the most (heck, I don’t even RP the mesmer and she’s 80 now).
The guardian is super helpful to groups and super sturdy. She doesn’t dps worth a darn, mind you. Still, if the goal is to win the fight, she will prevail over time against anything but a boss (and frequently has to suicide so the rest of the group can rez). She’s my only character with a lot of Healing Power and it makes a huge difference when I use my heal.
The mesmer surprised me with survivability and, oddly, with underwater power. I actively choose to go underwater on her for daily kills because mobs simply melt under her trident assault. On land, she stays up most of the time. Once you get the hang of the F1-F4 skills, and the clone/phantasm utilities, you can have multiple yous dazing and confusing the enemy almost nonstop. Also, when downed you have some amazing skills to get back up again.
As I am almost 100% PvE, my thief has proved less handy. He’s great for exploration and mobility, less good on dps (though that may be me not knowing what the heck I was doing in picking traits, as he was my first character). He’s still my main for RP and achieves, but it can be frustrating getting downed so much in dungeons and world boss fights.
My norn Ranger is only level 21, I’m not enamored of pet classes as a rule. I don’t have a feel yet for what she might be like at higher levels though I have several Ranger guildies that do very well.