I agree with you, the best location, as UrbanBoy has pointed out many times is directly on the property with the Salt Palace, the exact corner you pointed out Makid.
I think the next best location would be on either West Temple between 100 and 200 S or demo the Shilo and place it on that corner. Those three locations have the best connectivity and are still in the heart of the city.
Another decent location is were the Wyndham currently sits on S Temple, but there is nothing inherently wrong with that hotel so there is no need to tear it down to build another one.
The Japan town block is a bad idea, while there is room on the SE corner of that block, it doesn't face any main doors, and faces the loading docks. If Japan town weren't still there, a little bit. I wouldn't be against placing the hotel on the SE corner of 200 S and 300 W facing 300 W. That is close to an entrance also close to ESA and is currently a surface parking lot.
Here is something I had proposed over a year ago. I had wished that Marriott would have taken advantage of the CCC construction to remodel the downtown Marriott and purchase the air rights above Nordstrom and build either of these two version. I think I estimated this additional tower would have brought the downtown Marriott to 1,000 rooms. I didn't place the new towers in the south elevations, just the west, sorry.