Varus got with the newest update an incredible hybrid damage boost on his ult+Q. Its masteries and abilities order are constructed around his new benefit. The new Essence Reaver allows him to regenerate mana on the high crit chance meta. In favor of this, the new Runaans Huricane fits the demand on crits and the spread of his passiv W perfectly! Also you can build your runaans fully before building out bersekers due to the new 5% movement speed buff on it, and the bersekers higher price makes them insufficent in this point of the game. The rest rounds it up, last item melts annoying tanks.

After your Infinty edge is built, the rest will be built situationally.
-When Tanks starts to grow fat, build a last whisper before going on.
-When you get behind get some lifesteal

Your Q does a ridiculus amount of damage after ulting! Use this to your benefit! Choose hybrid masteries instead of lifesteal for this, you can make this compromise due to health pots are way more powerful and supporters heal more now.
The stacking on his W before sniping becomes more significant, so does his damage.
The core Runaans Huricane makes him to spread his W everywhere to prepare the damage on your snipes.

Make sure you choose Natural Talent for hybrid damage output on Varus's passive and the ultimate damage output on his ult combo.
explorer instead tough skin gives you the mobility that Varus urgently needs.

If the W's theory around this build seems to be less powerful, try the second page's abilities order on 'E' and the Lifesteal mastery instead of hybrid.
As Berserkers got more expensive try to consider buying Ionians Boots of Lucidity to have your 40% CDR at mid game, so your ult pops up at every possible situation to gain his new power spike more often. Death's Dance brings additional 10% to the overkill of arrows.