Attack Damage - Armor(mostly stuff like Thornmail or Frozen Hearts though of course there are many exceptions and items.)
Ability Power - Magic Resist(Negatron Cloak, Hexdrinker, Maw of Malmortius, Abyssal Scepter, many other items of course)
Burst Damage - Banishee Veil(This will generally throw off a person's entire burst combo considering you don't get hit by a skill before they start on it that is.) Another option is Guardian Angel's, if you plan on dying or being bursted down than this is probably best so you can rejoin the fight.
Being Focused - Zhonya Hourglass/Flash/Guaridan Angel
True Damage - More Health (not much more you can do with this.)
Invisible Champions/Wards - Vision Sight Ward/Oracles
(CC)Crowd Control(Stuns, silences, etc.) - Cleanse/Quicksilver Slash/Mercurial Scimitar/Mikael's Crucible/and items with tenacity such as Mercury Treads can be a great help in reducing the time of such Crowd Control.
Ganking - Wards of course! Though mia's or ss from other lanes are also nice to have. Or even the V to ping to fall back.
Counter Jungling - Either counter jungle back or place wards in your own jungle at main neutral camp spots, that way you and your teammates can catch them red handed and score a kill most of the time, as well as set back their jungling a bit.
Armor - Black Cleaver, Last Whisper(probably most effective overall since it ignores 35% of the target armor), Youmuu's Ghostblade, Brutalizer, and (optional) Blade of the Ruined King(which takes 4% of their hp away, doesn't reduce armor but its still quite nice to have overall.)
Magic Resist - Void Staff, Sorcerer's Boots, Haunting Guise, Liandry's Torment.
Healing Effect(s) - Ignite, Executioner's Calling.
(Pretty sure there are other thing's(in fact a lot) of things I'm forgetting but I guess this'll do for now, you're welcome to give your opinion's or even your own set of advice.