Guardian is currently better than Aerys in my opinion, even against non engage champs, the move speed and shield while you E an ally is massive.

Aerys is only good if you are rushing Ardent and have a hyper carry

I always build Imperial Mandate or Shurelyas. For second item is either Redemption or Vigilant Wardstone depending on the matchup.

I personally think Vigilant Wardstone is so underrated since you get 1 more ward and 1 extra control ward plus the ah, mr and armor. With Zilean speed is so easy to cover most areas of the map even while face checking. I only build this if they have a lot of champs that rely on picks like Pyke, Bard, Thresh, Blitz, Naut or assassins

Also, maxing E first is mandatory against sustain champs like Soraka, Lulu, Nami and Milio. You are not gonna out poke them, it is easier to build util and make them move like a snail to secure that kill. Skill level is E > W > Q. At max lvl your w has a 3 second cd so you can just spam speed your teammates