I would dust your golden legendaries if you have a non-golden duplicate of it. Or it's just a bad legendary. They are cosmetic only so there is no reason to use a gold one over a non-gold one except for the shine/animation. As a f2per, I've dusted every single golden legendary (1 duplicate, 1 bad one).
Alexstrasza is a good legendary, but since you have a non-golden one, dust it.
Thalnos is a pretty good one to make. Grommash too but he is a class card so much more limited on where you can use it. Ysera is strong too, for more control oriented decks.