I'm a fan of maxing all sorts of Mitigation and CDR, then just trying to catch every enemy champion with my face while spamming E, slowing with Q and keeping your W up as much as possible. If they don't focus you, you will eat everything. If they do focus you, by the time you go down your team has killed everything.
I like to take a Jungling path with this build, but lane works just as well if you play defensively (and know how to conserve some mana).
0/21/9 masteries taking:
Armor, MR, Strength of Spirit, Dodge, Nimbleness, Health, and the very last one that mitigates 4% damage.
Red runes, I've had success with Armor Penetration for jungling, else Magic Resist or Armor.
Yellow runes, I'd opt for Flat armor specially if jungling.
Blue runes, if you feel you will have trouble obtaining CDR, take some flat CDR runes, else Magic Resist.
Quints, Health, Armor pen, CDR, more Armor, its your choice really.
Exhaust/Smite | > All these work depending on your choice of lane/jungle.
My favorite way to build him so far is:
From there, its pretty team dependant.
Good choices for Magic Damage carry on the opposite team:
~Force of Nature
Some choices for a Physical Damage carry on the opposite team:
Try to at least get either FH or Randuins as the CDR proves to be very useful.
As always, Aegis and Guardian's Angel are a nice way to get a bit of both worlds while still working on your mitigation.
If you find yourself needing a bit more HP (even though with a Banshees or Randuins and your base its pretty darn high), you can always opt for a Frozen Mallet or Sunfire cape which will add a bit of damage in one way or another.
If you feel confident and think you can avoid death pretty well, a Leviathan never hurts, specially if you can get that mofo to 20. However, it isn't a very reliable way to get HP, use at your own risk.