League of Legends (LoL) Question: what is the perfect item?

Usually perfect item comes in mind with some obvious synergy with the champion. For example Kog'Maw does heavily rely on his Bio-Arcane Barrage. Even without items, it would be 80% of all of his damage. Now that we know what is the most important part of that champions kit we can start thinking about items. Bio-Arcane Barrage scales best with Attack speed. It can do well with critical chance and ability power as well but critical chance would not be as impact since it would not increase the damage of the ability itself and too much ability power would quite destroy the main point of the champion itself - shredding tanks. Now that we know, that we want mainly Attack Speed and Ability/Armor penetration, we can start thinking about items. If we would look closely at all different items we would soon assume that Blade of the Ruined King, Guinsoo's Rageblade and Runaan's Hurricane give us exactly what we need. More attack speed for more DPS, more anti tank damage and more survivability from Blade of the Ruined King's speed boost and lifesteal since Kog'Maw has no movement speed ability. If you would do this heavy thinking with every champion in the game, you would soon be able to guess most of the items they usually use. Just ask yourself - What is the reason why I picked this champion and What does he/she do best? How can I even further increase it? How can I minimise his/her weaknesses and What items are effective at the moment?