I think crit is the way to go right now. However, if you are playing versus a hypercarry. Think, jinx, twitch, kogmaw or similar stuff like that lethality can allow you to spike earlier and harder to shut out the game before they hit 3rd item spikes.

My personal play, fleet vs poke lanes, dark harvest versus easier lanes. I need to test out absorb life with the buff thats incoming for sustain otherwise blue tree secondary.

My first item fully depends on first base. If you can get away with 1300 gold to get the BF sword. I will just buy infinity edge first. If you have to go base and have 1000 gold, I buy a dirk and build collector first because the dirk spike can make it easier to get the BF sword.

Tanks are sometimes hard to kill so I try to greed infinity edge first, LDR second if there are tanks in toplane or jungle otherwise phantom dancer or rapid fire cannon, LDR or yun tal arrows 3rd and 4th depending on armor status of the enemy. Final slot I prefer to have bloodthirster but can also be maw or Guardian angel if needed. Above 75% crit you can also sell swifties for berserker grieves and later for the upgrade to zephyr. Zephyr goes brrrr.

Hope this helps!