What counters Varus mid?

Varus has no mobility and no sustain or CC pre-6. His poke stings like heck, but assassins like Fizz and Ahri can ruin his day, and high sustain champs like Kayle can weather his poke and force him out of lane. Bonus points if you have a skill that can dodge his Q (Playful/Trickster).

Fizz/Ahri/Zed/Yasuo/assassins in general. He has zero mobility and depends on long range poke to stay safe. Pick a highly mobile assassins and it's pretty much a free win. I still have nightmares about playing Varus into a fizz, he would miss in ult, or I could flash and avoid the fish, and he would still kill me in-spite of it, and me having an early kill because of my jungler. Some match-ups just really don't matter how well you play.

Mobility based assassins who have easy access to items like Zhonya's.

Oh boy Talon is Varus' nightmare.

AD caster lucian surprisingly does well against him. Since he will dodge anything Varus has to use.

Then of course most burst mages