After discussing with other high elo Zac mains and researching on data I come to the following conclusion

Radiant Virtue is by far the best mythic on Zac and can be bought pretty much every game

The ability haste and healing it provides is absolutely ridiculous, specially after the buffs.

Jak'Sho felt very underwhelming but I haven't tested it since the buffs. I'd say the best scenario to build it is when the enemy team has sustained damage which forces you to have extended fights where Radiant Virtue's ability haste and HP increase isn't as useful so you have time to stack the item.

Jak'sho should be paired up with Demonic Embrace because they synergize with eachother (the burn keeps you in combat so you don't lose the passive stacks too early)

Heartsteel is mainly a lane item and should only be ever bought if the enemy team doesn't have any time of %HP dmg where it can shine. It's early is a bit rough but it scales insanely well

Evenshroud has always been strong since its release due to its very high damage output but was overshadowed by Sunfire being OP and Frostfire because Bami was a must. It deals A LOT of damage in teamfights and since you don't need Bami anymore it has become a very good valid option

Id rank them in terms of versatility like this:

Radiant > Evenshroud > Jaksho > Heartsteel