Playstyle and build can be pretty matchup dependent. I only really play him top lane, but I'll try.
For top lane:
Runes: Armor Quints, MR Glyphs, Scaling Health Seals, Hybrid Pen Marks
Masteries: 9/21/0
Skill Order: QWQEQR, R>Q>W>E
Item Build (Standard): 
Item Build (My Preferred): 
Starting Items: 
You have to build and play him differently depending on your lane opponent. Typical items I find myself buying early are: 
When trading in lane, I find that Q is the most efficient and it's easy to use after an autoattack to prevent retaliation. This is very good for bullying melee opponents and waveclearing. When trading with opponents, you want to try and avoid as much damage as you can by using your Q to interrupt autoattacks and W to avoid skills.
I find that it helps to let my opponents push toward my tower, since it leaves them more vulnerable to jungle ganks, which Maokai has the CC to easily set up.
As a jungler, I'd recommend using MS Quints, Armor Seals, MR Glyphs, Attack Speed Marks. Max Q first, since it's your best consistent damage and has a fairly low mana cost. I'd recommend building Juggernaut+Ranger'sTrailblazer, Frozen Heart, Locket of the Iron Solari. While this might put you a little over the CDR cap, Locket or Spirit Visage is still a better pickup than Banshee's Veil.
