That's a difficult question to answer
In real terms all instruments are as difficult as each other to play well, or professionally, but in general terms I suppose any instrument that requires YOU as the player to actually produce the intonation of a particular note. Instruments of the violin family fit into this as they have no frets in the fingerboard - the player must place their fingers in exactly the right place to achieve correct intonation. Producing a nice tone from the bow arm is also a feat in itself.
In the woodwind I would say the oboe is probably the most difficult to play, partly because producing a nice tone through a double reed is something even advanced players have to keep working on. A bad reed is also a bad thing - even for good players bad reeds can sound awful. People also think that on woodwind instruments you simply cover the holes and blow, but that doesn't necessarily produce a good note. The pitch of any note can be altered slightly by changing the shape or pressure of the embouchure, so once again the player is having to listen to their tuning.
Brass instruments require the lips and mouth to effectively act as a reed, but of the brass family the trombone is perhaps the most difficult as it has a slider instead of valves. It requires accurate positioning to produce good intonation.
OK, so I've mentioned about three instruments here. What do others think?