My Pistol Dynamics Custom XO model 45, bought for me by my loving and longsuffering wife on our 5th anniversary
Lost in a tragic boating accident (in case anyone would ask)
My Crosman Pumpmaster 760, most expensive if sentiment is the currency. It was my first gun. I still have it, up on top of a tall shelf. It still functions. The GF and I got it out last summer to hunt some soda cans that were in the back yard.
I could also throw in another long gun that has a sentimental significance. My Future Father-in-Law bought me a Mossberg 500 Field last year for my birthday. He and I have become pretty good pals, so that shotgun means a lot to me.
Other than that, I'd say my actual most expensive is my AR, but that has been a combined and constantly expanding investment; I have significant money wrapped up in my Upper and Accessories. Does that count?
My most valuable gun in my collection is my 1943 Union Switch & Signal 1911A1. It is all original and has the belt, holster and cantine that was issued with it. It belonged to my step mothers father who was a Staff Sargeant in the 82nd Airborne. He fought in France, Holland, and Germany. Its my most valuable not just because if the value of it but also the story that comes with it.