Viper

I've been trying unfair bots recently and I find the only time I consistently have a hard time is when I am versing viper.

What would be potential ways to counter it?

Silencer does the trick. You can orb walk him down with Glaives of Wisdom and Last Word disarms him, because Poison Attack does not count as a spell.

1v1 there are not many.. he's designed for that type of fight..
Slark would likely do a good job, but have a hard time laning phase
same with Obisidian, but do better during the laning phase.. <-- though this would be a farm fight, more than kill eachother
Naix with rage would pwn Viper.. and out carry him with items.. 1v1, get a blink dagger.. sorted..

QoP would probably do good too.

Thanks for the responses, QoP didn't go so well for me but that was probably due to my experience.
I thought Kunkka and Silencer went quite well. Obsidian went reasonably well but I'm horrible with last hitting and denying -_-

I've found Abaddon to work really well against Viper. The important thing is not to let viper get early kills. So if you are laning against him take fewer risks. If he doesn't get those early kills he usually won't snowball into an unstoppable monster. Also I usually do very well against him mid with Disruptor perhaps not the most obvious counter but I find it effective early game as viper is quite slow you can trap him in the Kinetic Field fairly easily, the biggest problem there is convincing your team to let you mid as Disruptor.

I was in a pratice game once with Vengeful Spirit and we had a Viper against us in mid.
it allready killed our mid a few times and my lane was doing good, so I decided to gank.

I had Boots of Speed and a Medallion of Courage (medallion is THE BOMB for VS)

by the time I arrived our mid was back to base and the viper was 2 lvls higher than me
(I believe it was 8 against 10)

so it was 1 on 1 (I came from te back so I initiated)

I used Wave of Terror first, than I used Magic Missile, and than I used Medallion of Courage by that time viper was almost half HP.
and with -4 armor from 3 points in wave of terror + the -6 armor from the medallion, viper was a goner.

I was very surprised myself, but apearantly that's very effective against viper. Stun and armor reduction.

the most practical way is to suggest Viper to your bot allies, actually.

But than you still don't know how to deal with him when you catch him in public.

True. My answer was based on bot matches though.

As for me, Batrider went quite well. Though then again he does well against most mids. Dodging Viper Strike with Phase Shift as puck is great too, not to mention Puck is a great mid anyway that relies more on his long range Q so no risk of being orbwalked. Shadow Fiend is risky but he differs in the part that he can beat Viper at level 3, which most mids can't really do.

The trick to outlane viper is keeping the lane equilibrium, always keep the creepwave around the slope from your high ground towards the river to ensure easy lasthitting and no Qs on you.

Never autoattack. This can help the above tip very well and if you still find the lane being pushed, just autoattack your creeps from when they're half health to get the lane to your favor.