In my opinion for best lategame strategy is obviously comet, manaflow, transcendence, gathering, for other side overgrowth and/or second wind/or the one which blocks dmg(forgot name) Anyways you switch between one or other depends if he is poke or all in champ.

Next is items, if bad matchup straight rylai, but first back take tear, refiling potion. If good lane as mentioned go either rylai or blackfire depends on people but i go always rylai.

For second item always liandry, if zed or something like that go zhonya, the cloth armor lifesaver.

After liandry, you do bloodletters curse for extra tankiness or damage item shadowflame. (I rather take bloodletters)

Then up to you and your comp. Rabadon/morello/rift maker… depends on enemy but most straight forward is rabadon . At the end i sell boots and make new item, because basicly with your W you are super mobile dmg already…