Zilean is one of my favourite champions and I have played him many times in normal, ranked 5's and solo queue over many accounts. The best way to play Zilean is to play him as an earlygame-oriented AP caster. I've been playing AP Zilean for over a year now and it still works wonders when I play him with this build.

First of all, he offers a ton of utility. His speed buff/debuff is likely the best in the entire game and his revive is absolutely amazing. In a single fight, Zilean can speed buff/debuff two allies or enemies and revive a teammate for upwards of 2000hp. Also, unlike most AP casters, Zilean does not need an ultimate to unleash his full burst potential. A simple double bomb combo can EASILY take out the majority (or all) of a squishy's hp. This makes his harrass in lane extremely deadly and since it is not a skillshot, zoning with Zilean is incredibly easy.

Zilean's dominance in lane allows him to shut down almost any opposition and come into midgame strong and possibly end the game at 20 minutes. However, MAKE SURE YOU GET BLUE BUFF! If you go the Deathcap rush route, Zilean is extremely mana-hungry. It provides the most power but also is the most blue dependent build. If you know you're not getting blue buff for some reason, opt for the second build.

Boots + 3 Health Potions is the only viable start with Zilean. He needs the movespeed advantage to be able to plant his bombs.

Aim to recall when you can get 2 Doran's Rings. If you are having trouble, buy 3. Get a chalice too early on. If you are having a very difficult or passive lane, you can skip the doran's rings to get a catalyst and rush a Rod of Ages. I don't like going down this path because Zilean is most for early-mid game lane control and ganking potential. He falls off pretty hard and rod of ages is more a late game item.

Here is where I usually finish my sorc boots and start building up a Deathcap. By now you should have been snowballing off your lane and ganking whatever side lane needs more help.

Finally, grab your late game situational items. I usually always get zhonyas hourglass and a void staff if the enemy has a good amount of MR (and they usually do). For a final item I'd recommend deathfire grasp for more damage or banshees veil for survivability, though most games dont last this long.

The masteries are standard for an AP caster. 21/0/9 for pure damage and extra buff duration.
You can also go 21/9 and grab MR/HP masteries if you know you're going up against a hard lane.

The runes are a staple 9 magic pen marks and 3 flat AP quints. Take AP/lvl blues for maximum damage output or flat mr/mr scaling blues for lane domination or vs a strong burst champion. I prefer getting armor seals for survivability but it's all personal preference.

-Harrass as much as possible in lane. Try to push them out of lane with your long range bombs
-Slow them when jungler comes in for a gank, it's pretty much a guaranteed kill
-Gank whenever possible. Zilean is extremely mobile and has extremely deadly ganks
-Make sure to put your team ahead and win the game ASAP!
-Ask your jungler for blue. Zilean is very mana dependent
-MAKE SURE you die when you pop revive on yourself. You respawn at almost full hp.

Unfortunately I can't show off with screenshots like other guides, but Zilean is a really strong and fun mid laner. Try it out for yourself! If this guide helped you don't forget to rate and comment.