Good class to level with engineer
A second engineer.
There really isnt any bad synergy between different professions, certainly not for leveling. So i’d say pick what you like and it will no doubt work just fine.
Just keep in mind if you run stuff that knocks mobs back. Just because its of cooldown, doesnt mean you absolutely must use it. Your friends will hate you if you keep knocking mobs out of their attacks with something like Overcharged Shot.
Anything. No, seriously. An engineer can play any role, any specialization, and do well to amazing at it depending on level and skill. Find out how he plans to play, and choose something that complements that.
In general, the best would be another engie or an elementalist: each of you can focus in one area of combat, and have completely distinct skillsets. Rangers also work well; a friend and I leveled engie/ranger together to great effect. It’s a lot of fun to set up a turret nest, then have a pet lure mobs in to die That said, anything can work.
I wouldn’t recommend a thief, but more because I don’t enjoy thief PvE than because it doesn’t go well with an engie. Warrior PvE is very strong, and can bring the pain, while mesmers are strong everywhere due to their sheer amount of utility. Necros are at least decent right now if I recall correctly, in both condition and critical damage builds. Guardian is in a wonderful spot right now, no real complaints from or against them, but you already said no to that. Rangers are weaker than other classes, have always been, and likely will stay that way simply due to class mechanics, but they are still a ton of fun and are certainly playable (just weaker in some areas).
Ferguson’s Crossing