Champions that sustain easily work well. Ranged champions also tend to beat him easily, but it's harder to coordinate to let your AD carry go top, so Kennen, Swain and Vladimir are more likely to be the ones you'll use for that.

Olaf

Try this sight. Not all of them are right, but there in the ballpark usually.

http://championselect.net/index.php?r=champions/index

Gangplank can counter Garen very easy, farming and doing damage. Things become really easy if you buy a Philosopher's Stone.

Someone that isn't afraid of silence and can out-damage him (Irelia, Udyr) or someone that can kite him (Yorick, Kennen).

I once beat a Garen top lane with Ashe by maxing Frost Shot. It might have been the most fun I've ever had in this game. ^_^

uh malp he counters him easily. I mean he can use his q to slow him and then just run away.

I play both Garen and GP in a regular basis, and GP does not counter Garen top. Sure, Q spam on Garen sounds great in theory, but in practice what usually happens is that your mana regen and Q damage won't be able to keep up with Garen's tankiness and sustain.

Teemo on the other hand is a very solid Garen counter. You kite him all day long, harass with blind and toxic shot, and shroom the bushes to prevent him from camping there.