It's fun seeing posts like this, really. I play Zilean as a mid laner, and have an 80-90% win rate on him. Why? He does great damage, and wields powerful utility while being safe and relevant throughout the entire game. His laning is pretty strong(pretty much everyone you fight is a skill matchup), roams are very strong, and his teamfight is very strong.
So let's talk about mid lane Zilean:
His passive:
Have you ever maintained good cs, and killed your laner? If you've done it correctly, you should be two levels up on them when they return. If you're Zilean, that becomes three levels a good portion of the time. The hidden power of his passive is that if you can manage your lane correctly you can facilitate an increasingly wide level-gap between yourself and your laner, making yourself nearly unfightable for your opponent, and for most of the enemy team when you're levels ahead.
His Q:
I see a lot of people complaining about this, and I do not share those complaints. For people who can land his Q, it's better than ever. An AoE 2k+ damage nuke with a 1.5 second stun that's on a four second cooldown late? Sign me up, I don't care if I need to work to land it now. I personally can land these in lane on most people, and when I can't, I aim for low hp minions since a double bomb on one of them will instantly detonate both and stun, which is just as good as landing it on the actual laner, better in some cases since they instantly take damage from both. In the event they don't stand near minions, and I can't land them on the laner, I can wipe almost the whole wave with two bombs, if nothing else I can farm up and still be godly in teamfights. Early game two bombs will usually chunk 50% of your opponent's health depending on who they are, and upon completing almost any AP item two bombs will instakill caster minions and bring the melee minions very low(hint: AA them low then just wipe the whole wave for easy cs)
His W:
The skill that makes your Q spammable. The skill that makes your 99% slow almost permanent. Do I even need to put anything else here?
His E:
This is a 99% slow/speed buff that can be maintained almost permanently with W because by the time it times out, it's ready to be used again. This skill slows as much as Thresh's ultimate, and can be spammed.
"Oh gee that Udyr is sure getting close" Hit E, casually stroll past, stop to get that minion wave, continue escaping
"Oh hey my jungler is coming in for a gank" Hit E, instantly burn their flash, probably still get the kill
His R:
Let me start this section by saying his ult is on a 36 second cooldown late game. An extra life, every 36 seconds. This not only makes Zilean very safe, it makes his allies very safe. You can speed them up, slow the enemy, stun the enemy, and with your ult even save them from certain death.
"Zilean is totally weak against assassins"
Lol no. Please allow me to tell you how to deal with assassins:
Can they kill you in under 3 seconds?
If the answer is no, use Time Warp and walk away.
If the answer is yes, hit R, then do the above when you get up, or fling two bombs and one shot them, your call. In the event you aren't level 6, play safe until you are.
Congratulations, you survived where many other mages would've died! Unless they're super fed, or you put yourself in an absolutely awful situation, you shouldn't be dying to assassins. Even if you can't kill them, Zilean is well-equipped to survive pretty much everything. I mean honestly, the magic trick to insta-winning lane vs 90% of assassins is having your jg stand by, and when they waste their gap closer just E them for the free kill as your jungler walks in.
It's fine with me if he's underplayed, probably my favorite mid laner because of how adaptable and strong he is.