In many respects, while on the face of it a comparatively simple question to answer, it isn't!
For me, having been in the extraordinarily fortunate position of having owned some of what I would consider to be amongst the Pick of The Crop, I think if you don't mind, I'll separate this into 2 categories.
I had an absolutely beautiful old Martin D28 that I adored, sadly it was nicked, so I replaced it with a Martin 00028.
BUT, the most wonderful sounding acoustic I have ever played belonged to a very dear friend. That was a hand made in the UK Fylde Oberon. It sounded like a Concert Grand Piano and was an absolute delight. He still has that guitar and the older it gets (Now coming up for 35 years old!) the better it sounds!
Easy, I was a Gibson Nut through and Through so as mentioned by Mr J.E. it would be my desert island guitar for me as seen in my avatar. My gorgeous 54 Gold Top Re Issue. I have owned and played many Les Pauls, but nothing has ever been quite the match of this one.