1. Long Sword or Doran's Blade or Rejuve Bead + HP pots (1 with the swords, 2 with the Rejuve Bead)
2. BF Sword
3. Boots lv1
Then depending on what is happening I go down a few paths.
4 or 5. If I am doing well (5+ kills) then I try to pick up a 2nd BF Sword so that Heartseeker and Spear hit really hard. I also decide which boots to upgrade to (Treads for Magic and CC heavy opponents, Tabi for Physical DPS). Then go Stark's and upgrade my BF Swords to Black Cleaver and Bloodthirster or Infinity Edge.
4 or 5. If I need some survivability I will add Stark's to help myself and team. I also decide which boots to upgrade to (Treads for Magic and CC heavy opponents, Tabi for Physical DPS) and then try to pick up a 2nd BF Sword. Then upgrade my BF Swords to Black Cleaver and Bloodthirster or Infinity Edge
6. If necessary I get Aegis of the Legion to help push and survive team fights. Alternatively Banshee's Veil or Frozen Heart work well in the last slot.
I used to build Brutalizer first but found that if I just waited a few minutes and a few minion kills I could pick up the BF Sword and clear creep waves faster and all my abilities hit harder because of it. With my runes and masteries I have a base armour penetration of 30 so additional armour penetration isn't necessary until mid-late game.