I can potentially see the warrior, the necromancer and thief, and maybe the mesmer as being great 1v1 classes.
Others? Too many trade offs.
Elementalist is very squishy naturally.
Ranger is squishy and relies on wonky pet.
Guardian is squishy, without much spike damage.
Engineer does not have many options. Turrets are a no no. Gadgets are not damage focused. Elixirs are support. Kits? Perhaps, but the numbers seen on em suggest low damage.
I'm afraid I just simply disagree with you. Guardians are anything but squishy. And they have very moderate damage that can be sustained for a long fight as they are very survivable. Rangers have medium armor and health, which means they really aren't squishy. They do pretty sick single-target damage which makes them better at 1v1. The pet adds a 2nd enemy that your opponent must defeat.
Elementalists are only squishy when they only focus on damage. I did some theorycrafting and made an Ele that I suspect would be very survivable. Just gotta focus less on Fire and more on Earth. Something I haven't seen any videos do.