CC-Zilean is best at controlling the whole battlefield and is (atleast for me) the supp who has incredible high impact on the game. He is able to engage very well and to split up targets from their team. Furthermore he is able to stun several targets in tf and can save the squishy or important allies. Additionally CC-Zilean helps AD Carry´s just like Jinx or Ashe which do not rly have an escape to reposition fast.

In lane: In the early lvls (1-2) feel free to poke the enemy adc a bit with your AAs and your q without wasting to much mana. With lvl 2 you might stun the enemy supp with your stun.

COMBOS: Your main target is aswell the dmg dealer ofc but there are further possibilities to get him :

1.) E->Q->W->Q->E slow him, stun him slow thats huge cc that can be used to some serious dmg 2gether with your adc
2.) If the target is far away then E yourself->Q->W->Q->E the target so others may close the gap
3.) Q->W-Q on minion/or supp such as Braum e.g. next to target will aswell end in a stun to the target
4.) Q on your ally who engages W->Q on the ally to land a safe stun
5.) Speed Burst engage combo:
Activate Talisman and Righteos Glory ->Rush in; if you are still to far away use e on yourself try to stun a target . Activate Righteos Glory again (and further items such as Randuins) and use 2nd e on target or approaching allies.
This combo can be used aswell to speed up the whole team and the tank e.g. -> equal to Sivir ult. Very effective!
6.) Further usage of abilities :
1.)Q->W->Q into a bush when u see enemies incoming that dont have vision
2.)Q in general gives vision and does dmg so use it to zone enemies being aswell afraid of being stunned and to get vision of bushes
3.)Q->W->Q when there is Zhonya activated or GA popped out to insta stun them

I will add some videos later on

CC-Zilean provides lots of CC. If u have Zilean you should pick Champions that might have a
lack of CC, but that deal lots of dmg. Ofc u still should have a tanky frontline but Zilean is able to have enough impact to provide reposition in the battlefield and save allies with his ult. Zilean is especially good vs. Teams that do not have dash or jump escapes but even if they do have those champs or they got Morgana Sivir Botlane which is very CC resistant you can still speed up your jgl or your adc to flee or to go in and deal tons of dmg. With these posibilities
to take impact on the game Zilean is one of the best supports in my opinion

My Ingame Name is Zzeetz
if u want to see my stats especially with Zilean look here :
http://euw.op.gg/summoner/userName=Zzeetz
I am currently 74th best Zilean in world