What songs have a smooth acoustic guitar lick?

Acoustic guitar? Listen to some Doc Watson.

Tony Emanuel, Chet Atkins

I don't know if you'd call him "smooth" but when Jerry Reed played acoustic instead of his usual electric. :) :) :)

I'll just name some what I think are very good acoustic guitar artists, you can find your favorites: Steven Stills, Taj Mahal, Mississippi John Hurt, John Hammond, Ben Bolt, Donald Dykes, Jerry Garcia w/Dave Grisman, Guitar Republic, California Guitar Trio, Jesse Cook, John Williams, John Renborn, Pat Kirtley, Scott Adams, Sonny Chillingsworth, Led Kaapana  & a whole host of slack key players  & then there's Django Reinhardt & a whole host of players who've adapted his styling.

There are the acoustic blues roots players.. Skip James & Robert Johnson come to mind.. A lot of pop artists play acoustic on some of their stuff.. Jackson Brown, Neil Young  Country pickers, the best have already been mentioned, but I might add Merle Travis & Jimmy Dale Gilmore  There's jazz guys, you might find Charlie Hunter interesting, but most are primarily electric players.. Dang I think I could sit here all day, but enough.. Enjoy!

A good guitar player can add his licks to any song.

Candy Dulfer & Dave Stewart - Lily Was Here:

https://www.youtube.com/watch?v=_5kKo2_2MzI

And a lot of stuff by Al Stewart - look for the live performances or stuff with this guy:

https://www.youtube.com/watch?v=BnXt80Xvq8E

(Skip to about 0:40 in this one)

https://www.youtube.com/watch?v=w4UYsInNsFQ

Look for Rick Beato on YouTube. He has done several videos about acoustic guitar riffs.