Sceptile to me just seemed like a typical grass type pokemon in that it was fast, but could normally be killed fairly quickly, though in the last 2-3 gens I was proven wrong.
Fastest Subseeder outside Prankster, Decent special movepool, and coupled with Swords Dance for a physical set showed potential in generation four alone. Gen 5 was the peak in that it brought about unburden, bringing the ability to use a consumable item to make it nearly impossible to outspeed outside of priority. My personal favourite set became Sub-Petaya, as it caught out people who expected a standard physical set.
Others that have surprised me are Absol,Clefable (particularly now!), Staraptor (expected your usual first route bird, but close combat), and I was also pleasently surprised with Escavaliers sheer power with a Choice Band slapped on, along with Trick Room support. I also believe Goodra deserves a mention, as once again it can be so versatile with 3 excellent abilities, (although hydration has lost some usefulness with weather nerf)