There are 3 ways to build Varus (4 if you count AP Varus but that's more of a for-fun build):
- Attack Speed / Crit - Core items are Essence Reaver, Runaan's Hurricane / Statikk Shiv, IE.
- Lethality - Core items are Manamune, Youmuus Ghostblade, Duskblade.
- On Hit - Core items are BotRK, Rageblade, Runaan's Hurricane.
Currently, the Lethality build is the most optimal. This is because Varus's strength right now comes from his poke due to Q and E + Comet. Lethality grants you an early power spike compared to the Crit and On Hit builds which take more time. Also, due to the absence of armor runes, lethality is a bit stronger and got buffed (but it's about to get nerfed).
That's also why you see that MF and Jhin are also quite strong at the moment.