If in Zagreb I would opt for the DT (and will do so twice in September and October): It belongs to Hilton, I get my benefits, points and the stay helps in maintaining status. Locationwise there are better options. However, the DT is not that far away from the city center and the bus terminal (airport bus) is only two stops away. I find the Zagreb tram network reliable and comfortable enough to make up for this slightly remote location from the city center.
In my view, the Westin is as centrally located as is the Sheraton. For a more central location I will try the Palace next time (yes, BotB, the Esplanade hotel -built for passengers of the Orient Express those days, is on my bucket list too, but unfortunately neither points nor miles for that...).
Back to the DT Zagreb: I found staff very accomodating, the room great and breakfast very good (2 nights' stay). So I am looking for my two upcoming (1 night) stays.