^Giving it to Ray (or perhaps Ray and Gary) makes the most sense, I agree. Ray was the foreman of the ranch for 20+years before leaving, and worked it before that.
And I do agree it is somewhat odd that Bobby would be giving up on the ranch seemingly so easily, but keep in mind too that he always loved Southfork as a home for a big family. Right now it's just he and Ann who live there. Well, and J.R. too... but in the glory days they had everyone -- Jock (and later Clayton) & Miss Ellie, Bobby and Pam, J.R. and Sue Ellen (and later Cally), Lucy, Jenna, Charlie, John Ross, Christopher, James & Michelle, April (though, briefly)... and Ray and Donna lived in their house for years just down the road.
I also have a hard time believing Ray and Lucy aren't staying at Southfork when they visit either, given their relation to the family.
So I can sort of see why he'd be disillusioned with the ranch after living there so long with just Christopher and Ann. You live in a big house with lots of people and then move out/ they move out, things start to get very lonely, even if you still have someone there with you. They touched on this particular characteristic of Bobby's in "J.R. Returns" but having moved out of a big house where I lived with my moderately large family for many years, the first time I lived on my own was somewhat disheartening if only for how quiet everything suddenly was. So I can believe that reasoning if that's where they're going with it.
Anywho, sorry to ramble... I do agree though that despite his not being a full Ewing (and not at all a Southworth (Miss Ellie's maiden name), Ray would be the ideal successor to be owner of the ranch.
Keep in mind also that Miss Ellie didn't bequeath Southfork to Bobby in her will. In the final season of the original series, Clayton comes back to Southfork (he and Miss Ellie had left the previous season to travel the world) to literally give Bobby the deed to the ranch himself, as per Miss Ellie's express wishes that he be the official and legal owner of the ranch -- while she was still living and of sound mind. Unsurprisingly, J.R. was not thrilled then, either.