I recently started playing zilean and I have to say he is a very good champion.That early game harass is so strong with his double bomb.And if you get fed and start doing a lot of damage and you get focused you ult yourself with all your ap and your fine!So why did he lose all his popularity?One game I got flamed for picking him, another for going mid as him.

He doesn't work as a support because you need huge balls to play him. All poke, no sustain. I mean hell, Lulu has a shield at least.

He lacks enough damage for people to want to put him mid. So they don't like him there.

I just play him mid as a giant **** block to all the assassin mids I'm seeing, like Diana and Akali.

I have actually had some decent success playing zilean has a support against supports that dont have much sustain such as taric, leona, nunu, and janna. mostly because he can harass the adc health every time they try to cs. He's also very useful in team fights keeping your most important players alive. I don't know why he isn't used more.

I tried him
I liked him
I was bored the whole time

He was popular for a few weeks because M5 used him in worlds, then he fell back to normal.