Since this has resurfaced, I'll answer it.
Meditation from Thais is the thing I've already 'played' to a certain level which I'd like to play really well...because playing the notes and playing it well are worlds away.
My favourites in general are slow, expressive pieces like Schindler's List, Rachmaninov Vocalise, Chopin Noturne in C# minor.
On a short timecale I'd like to play Praeludium and Allegro well, because that's the main thing I'm bashing away at at the moment, and whilst I'm not sure Kreisler's quite my cup of tea overall it would just be such fun to be able to play a piece like that without it seeming so hard! not really possible I know!
On a very long/when pigs fly timescale, my favourite concerto I think is Beethoven, but what I most want to be able to play is solo Bach...can't decide between E major and D minor....possibly D minor just has it.
That was a rather long reply and much more than one piece!

whoops.
Better get practising.