At lvl 7 you can take any depending on the situation and preference. I would suggest W when you need a little more health and zoning, Q is just good and helps you threaten people with your dmg, and the passive is good if you need better mercing or wave clear if your team is lacking. At lvl 13 there are options depending on the types of dmg the other team does, but i would suggest almost always taking the E upgrade. At 16 you can either take the E or the spell shield depending on if you need the spell shield.

Bunker is an underrated ability, it allows you to keep your back-line alive against a dive, a Fenix ult, a pyroblast, and it gives 25(or 50 at lvl 20) armor after exiting bunker which can win a fight by itself.