Not Updated For Current Season
LeBlanc, The Deciever
This is the build I found most affective for my play style with LeBlanc.
I like to start out with Boots of Speed and Health Potions X3 because I like to have that early edge to harass or avoid harassment. Since LeBlanc has a good range and quick cast/animations it makes it quite easy to be aggressive and score early kills. Being faster than your opponent is an advantage and pretty self explanatory as to why... HP pots for staying power.
Next I like to grab Mejai's Soulstealer. If you scored a few early kills or were sufficient in harassing your opponent you'll have a good cash flow and be a level or two about them. This frees you up to gank side lanes. Thus obtaining more stacks to your Mejai's. :D
Completing you Sorcerer's Shoes should be your next priority. By this time, the champion you're dominating up mid has either switched lanes or picked up some MR to try and cope..
If it's the latter of the two MP will help with this.
I like to get Rylai's Crystal Scepter. HP is always nice, AP and a slow are great! If you're not great at landing your Ethereal Chains, slowing them first might help a little.
Void Staff. More MP. If the enemy team doesn't have much MR, then I'll often pick up Abyssal Scepter instead if there is a lot of magic damage flying around... and reduces what little MR they do have.
Time for Zhonya's Ring! You all know why, it's basically the best caster item around. Massive AP boost and an OH **** button built in. What more could you want?
Now I choose to get Lich Bane at this point because like I said before, I LOVE mobility so every bit helps. A little more MR never hurts. And with the amazing missile speed of your auto-attack it makes it quite easy to add that extra hit if the enemy champion has manage to survive your beastly combo.
I've chosen to not take Archangel's Staff or Rod of Ages because LeBlanc's abilities a very inexpensive to cast and I don't usually run into mana issues. The mana regen from my seals is enough for me. If I do find myself a little short on mana I just grab blue buff.
So there is no need to stack mana and thus Archangel's Staff is terrible for AP.