Morellonomicon: CDR, AP, Mana regen, and a decent passive. WTF else do you need?

Shurelya's Battlesong: CDR, health, HP and MP regen, and a good team-buffing active. GOOD ****, MAN.

Locket of the Iron Solari: This item is disgustingly good for how cheap it is.

Runic Bulwark: junglers often grab this, but on the off chance yours gets something else it's worth having.

Boots:
Mercury's Treads/ Ninja Tabi: if you need defensive boots

Sorcerer's Shoes: if you're winning

Aura items:
Will of the Ancients: This item is only worth buying if there are at least 2 other champions on your team who can utilize the aura, otherwise you might as well not buy it at all.

Zeke's Herald: The fact that AD mid and jungle is becoming common has made this item somewhat more worth buying. However, unless there are at least 2 other teammates who will utilize the aura this item isn't really worth the cost.

Shard of True Ice: If you're behind and need to keep the GP10 or your allies just can't seem to keep their mana up this item MIGHT be worth having, otherwise Randuin's Omen or Twin Shadows are better options for extra CC.

Situational items:
Athene's Unholy Grail: basically all the things Zilean loves rolled into one item. You can get this instead of Morellonomicon or in replacement of other items if you're fed.

Twin Shadows: Good item in general for an AP oriented support like Zilean. The active is particularly useful against stealth oriented champions

Abyssal Mask: Good if you have a lot of magic damage on your team and no one else has one

Zhonya's Hourglass: If you opted for an early armguard this is the logical upgrade

Randuin's Omen vs. Frozen Heart: Both of these are good armor items to have. Randuin's Omen if you're engaging on the enemy team, Frozen Heart if you're getting engaged on.

Mikael's Blessing: really only good against burst CC.