ad carries generally have the most inflexible builds. At endgame, they want to have IE, PD, LW for maximized autoattack damage. Everything else is sustain or survival (BT or GA/QSS/banshee's)
Option one: Boots/3 -> BF sword -> greaves/BT -> PD -> IE -> LW -> GA/QSS
Option two: Boots/3 -> 2 doran's/greaves -> BF + vamp -> Zeal -> IE -> PD -> LW -> GA/QSS
Option three: Boots/3 -> 2 doran's/greaves -> IE + vamp -> PD -> LW -> GA/QSS
generally any combination of rushing big AD then rushing LW, getting BT if you need sustain and IE if you're winning and want to get further ahead, or need to rush it to catch up. (I've actually won many games where I lost HARD early game but rush-farmed my IE while he went BT, since I outscaled him as soon as I got IE, i was way more useful) 2 doran's + greaves VS Bf sword is probably the biggest choice you make all game, do you need a bit more sustain and tankiness RIGHT NOW or more raw damage to push for lategame
compare this to a plethora of builds and build options for any other role (except maybe support, rush Gp5 -> build auras)