Is this for support?
Max Q against low-mobility laners or in a lane where you and your adc can bully them hard. But then, there are better picks than Zilean for that.
Usually, max R-E-W-Q. By rank 3 of E, you can slow any bot laner enough for an easy, guaranteed stun (unless they dash or flash). Once it's maxed, you have a long, reliable, very powerful CC chain going into midgame. E, Q stun, E again for 1.1 seconds of stunning and 5 seconds of essentially rooting them. As you stack CDR, you'll also be able to spam E on yourself for lightning fast roaming, and your reliable CC chain lends itself well to catching people out and saving allies.
Max W next, after which you'll have about a 1 second window between the end of your basic CC chain and the start of your next one. You can even stun people twice with 3 bombs if you time your reset perfectly. Don't bother maxing Q if you started by maxing E - the damage is only relevant in lane, while the spammable reset on your Q and E makes your CC chain that much better. You can also permaspeed yourself for roaming once E and W are maxed, with about 25% CDR.
When you're finally starting to max Q, you'll have enough CC from your other abilities to safely buy a Mejai's and start stacking assists. With just Mejai's and the FQ line, your maxed bombs will be doing very respectable damage in endgame teamfights, while you benefited from your more reliable CC through the early and midgame.