First off I would just like to say that I'm almost lv 28, so my final Spell Vamp total isn't at it's maximum. Only because I don't have the Quintessences yet. But this is what I'm going for, and all the items I have listed are the ones I use.

1. Crystalline Flask
2. Amplyfiying Tome x2 or Hextech Revolver (depends on how well you're doing)
3. Spirit Visage (Increases total Spell Vamp, HP, and M. Resist)
-Alternatively you could work on Zhonya's Hourglass instead if you're facing an AD Champ on mid lane. But Finish S.Visage for the build soon!!
4. Then build a Chalice and Morellonomicon for the Mana Regen and Cool Downs. Or build Deathfire for the AP and Cooldowns if you prefer.
5. Zhonya's (if you haven't gotten there yet) and Rambadon's Deathcap for AP/Armor.

Pros:
-Awesome HP Regen because of Spell Vamp.
-Pretty high AP late game, and quick spell combos (winning).
-Easier to chase champs/escape from fights if need be.
-Awesome Lane sustain during early levels on mid lane (can be used on bot lane).

Cons:
-Low AP early game
-Still Mana hungry early game
-Might not be as handy on bottom lane.
-Not as high AP late game for the defense, but the spell vamp helps if you have to defend (losing).

Build Hextech Revolver first for the inital base Spell Vamp. Start Building Spirit Visage if you're facing another AP champion, or doing well enough that you can afford to skip out on a bit of AP in exhange for Spell Vamp/HP increase. Other wise build Zhonya's Hourglass for the AP and Armor bonuses if there's an AD champ on mid, or if you need as much AP early game as possible. After either of those I build a Chalice, then the Deathcap for more AP. this is where a lot of your AP is going to come from anyways. Finish with the Morell-con for more Mana regen and cooldown reduction. Additionally you can replace this with Deathfire for even more AP if the opposing team doesn't surrender first.

Have fun!