Jax: "I cannot believe how hard this Janna is carrying. Good job! :D"
Kog: "How does Shen lose against a Janna?"
Shen: "She pushes me out, obviously"

So I mentioned this in a forum topic not long ago, but I feel like the best counter to Shen top is AP Janna (aside from just banning him). She's unkillable, and can punish him hard for leaving lane by pushing really fast. And if he's dumb enough to use his ult in sight of you, you can just knock him up / back and cancel it! When you pick AP Janna vs. Shen you say "screw you, you're NOT leaving this lane."

So I actually got a chance to test this. Saw Shen pick and said "I want to go top vs. Shen." I was actually last pick, and the Sona had originally picked Janna. Surprisingly enough he let me pick Sona to switch with him. Most people just think I'm trolling when I go AP Janna top. It worked as I imagined, I was actually winning lane against Shen, he was only beating me in a farm by a little but I got his tower down.

We actually were losing quite badly. Mid tower went down early, and bot was feeding. We were 2 inhibs down, but slowly we re-established ourselves, and started winning teamfights. I love the utility of Janna in teamfights! Even though she doesn't do as much damage as a more standard AP, her utility is invaluable, and now your shields shield for a LOT and your heal is quite noticeable. ^.^

Anyways, that's the most fun / challenging game I've had in a while, and I'm glad I got to try out my theory. This was just Normal Draft mode, but I think it's enough evidence for me to keep trying. AP Janna top as Duff put it is a "you can't win, but you can't lose" lane. You most likely won't be able to kill the enemy laner, but he can't kill you either and you're very hard to gank. I would suggest trying it sometime if you're skeptical!