Against mages like , , or teleport is a must, and against assassins and champs like , and you take . There are matchups where you can choose and both options are viable. For example vs , , , , you can take or and both are fine, but generally I prefer teleport, except against Zed, vs him I will always take exhaust. Only against i highly suggest .
If you take teleport you want to start with corrupting potion, (this leads into the ZEAL BUILD, which you take with the FIRST RUNEPAGE) and extensively trade with your lane opponent while spamming your corrupting potion charges. Loosing your hp doesn't matter, because as soon as you get 450 gold, you want to back, buy cull and teleport straight back to lane. Your next back will be Sheen and then you will proceed into triforce, into lucidity boots, into infinity edge into statikk shiv, into phantom dancer into lord dominiks regards, and then you will sell boots and buy essence reaver.
The SECOND BUILD is basicaly the same, but you don't start corrupting potion, instead you go mana crystal and a refilable potion, and you do not extensively trade in lane, you just farm. You will then go b, buy sheen, and optionaly cull, then proceed into triforce, ninja tabi, youmuus ghostblade, infinity edge, phantom dancer, and statikk shiv / lord dominiks regards, depending what they build, and finaly sell your boots for ldr or statikk, depending on what you bought first.