Well, you can’t got wrong with Steve Morse. Some of his stuff can be very challenging. I would recommend Modoc as a good place to start though it has an open tuning. You can find the tab at
http://www.stevemorse.com/tab/modoc1.jpg
For really challenging Morse pieces you can also try Point Counterpoint and Flat Baroque which you can also find on Stevemorse.com
Slee
Leo Kottke. Just about anything. He uses a lot of open tunings and the fingerpicking technique can be difficult. The Fisherman is not too difficult and fun to play. Little Martha’s a beauty as well.
Check http://www.leokottke.com/. The discography here lists most of his tunings.
If you’re not familiar with his work try out Leo Kottke Live (1995
Private Music 0100582132-2). Great stuff.
I second Watermellon In Easter Hay by Frank Zappa. Beautiful tune…not acoustic but, as mentioned earlier, may work well played that way.
Damn, got the (Welsh) spelling wrong, thanks for the second Jurph
For the record I think Bron-Yr-Aur is pronounced Bron-err-eye-er, hence the boo-boo.
Nurobath are you trying to scare the poor chap with Leo Kottke?
While I’m here, Watermellon In Easter Hay is a wonderful tune but I’m not sure you could arrange it for solo guitar. Go on, prove me wrong.
According to the book Hammer of the Gods it’s pronounced “brom-rar”. Are there any Welshmen in the house?
Bron-Yr-Aur Stomp (not the same song) and Gallows Pole are good choices if you want something more upbeat. David Bowie’s Andy Warhol is also a very nice acoustic piece.
Asturias by Isaac Albeniz (part of the Spanish Concerto) is the piece I sold my soul to the devil in order to play. (Funny, it also involved months of practise and attentive care of my fingernails - souls don’t go for much these days, it seems.)
Bach is always a winner: Air on a G String, Jesu Joy of Man’s Desiring and the enchantingly titled BVW 903 are all beautiful.
The Chilli Peppers Under the Bridge is also remarkably effective if you learn the backing (from TAB or whatever) and include the melody, fingerpicky style (you’d have to work this out yourself).
Thanks! It’s a gorgeous little gem, and I tend not to like the word “gorgeous” 
I haven’t been able to try out all y’all’s suggestions, but I have this thread bookmarked, and I’ll probably keep checking back here when I’m looking for new stuff to play. I think I’ll be occupied for a while, though…
I appreciate it, Cafe Dopers. 
Here’s an arrangement of St. James Infirmary that’s fairly easy but sounds cool. There’s a midi file too, so you can hear how it goes if you’re unfamiliar with the song.
Touching on Steve Howe, another great solo by him is “The Ancient”, which is in a collection of his pieces that also includes “Mood For A Day”. The actual tune appears within a larger, typically epic song by Yes; actually listening to it first, if you can, will be a great help in working through the many time sig changes.
I used to be able to play both, fluently, but sadly have forgotten them. 
But wait, there’s more!
In the same vein & era, there’s a very short piece called Horizons on Genesis’ Foxtrot album. I don’t know if a written score exists, but it’s probably something you could figure out by ear it’s so simple, yet sounds very cool.