I think we need some more info – “smoothness” is a bit too vague. Are you referring to tactile smoothness, as in the silky feel of the wood under your finger? Or to the overall physical smoothness of the finishing material itself (think French polish vs. a sprayed on lacquer). Hell, I’m not sure I’m even doing a good job describing the difference!
I think you’re referring to tactile smoothness – so I’ll comment on that angle. If that is incorrect, by all means let me know.
Tactile smoothness has as much to do with your surface preparation as it does with your finish material. If the underlying wood is not smooth, no finish will smooth it out and make it look decent. So it starts with the wood. To prep I like to plane and/or scrape as much as possible rather than sand. Most of the time, a planed surface will be as smooth as, or more smooth, than a sanded surface. If I have to sand, I’ll typically only go to 220. I’ll go higher only if I need to put an absolutely perfect gloss finish on the piece.
After that, I really like to use shellac. It’s easy to apply and you can do satin or gloss finishes with it. It feels wonderful under the hand, too. Unfortunately, it’s also fairly fragile. It doesn’t do well with water or alcohol exposure, so it’s not very functional for things like tables.
When I need a durable finish I’ll go with lacquers, polyurethanes, etc. But they’re certainly not my favorite.
On some things, a rubbed oil finish gives just the right feel to the piece.
It really depends on what I’m building. It’s the old adage – form follows function, and by extension, finish follows function, too.
Sorry if this doesn’t really get at the meat of your question.