99% of games - max your distortion (W) for waveclear and the shorter cooldown
1% of games - max your sigil (Q) for poke in lane and higher damage trades (basically if you can't afford to go in melee range of your opponent and you're farming under turret most of the lane phase)
Items:
Generally I build morellonomicon first. The CDR and mana regen is really useful, and since it has 80 AP you still generally have enough to blow people up with your ult.
Second I like DFG. You can also build zhonyas here if you think you need it, but DFG will let you 1 shot pretty much anyone without a full magic resist item at this stage of the game.
Third you want your hat. The only exception here would be if the enemy team is stacking MR, in which case void staff before hat would be better.
Final Build: (morello, DFG, hat, void, zhonyas, sorc shoes)
Some people may do it differently but I find that style to be very strong for me. I'm able to ult twice in a lot of teamfights due to rushing to 35% CDR and your distortion is up very frequently to reposition, escape, and assassinate. You have enough AP to kill any squishy 100-0 with your ult, and you'll chunk about half of a tanks health with your combo (which conveniently comes up every ~10 seconds or so). Good luck, leblanc is a really fun champion!