Now, I might sound really picky here, but I SWEAR there's this one part on Steel Vengeance where the train makes a "CLICK" sound and a noticeable jolt (it's around the outerbank into the structure on the 3rd "lap"). But other than that, it's incredibly smooth. This and Twisted Timbers are very on par with each other.
Other than RMCs, Phantom's Revenge and Steel Force were insanely smooth! I even rode on a wheel seat on SF, not even a single vibration. And a ride I really didn't expect to be as smooth as it was; Laff Trakk! I didn't think this ride would be too smooth, but honestly, it was probably the smoothest ride at Hersheypark.
For wooden coasters, Phoenix is the obvious choice for me. I knew it was going to be smooth, but not THAT smooth! Blue Streak in the front row, Thunderbolt (Kennywood), and Mystic Timbers are also very close to that top spot.