The oud.
End of story.
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Nah, I adore the oud, it is for me the ultimate distinguished classical instrument that can also easily turn into a casual folk instrument, the same way than violin, but I like oud way much (still love violin though!)
But also, other favorites:
Clarinet, nay, sarangi, daf.
I also play some guitar, I won't say it's one of my favorite instruments, but I like its versatility.
Also like a huge number of other instruments, but I have to chose favorites, so....