Ok. Based on ALL my stays at a variety of hotels around the Zocalo, Reforma, Roma Norte, Airport, Polanco, my best overall stay was at 4 points because I could just walk outside and walk anywhere without constant thinking about what time it is and who's around even though I was still precautious. From that hotel you can easily walk to reforma OR catch the metro bus in Roma at the Av Alvaro Obregon stations or the Durango station on Insurengentes to Hamburgo/Reforma area.

But the tourists sites are spread out around the city so there's no specific area that is best. Many people stay in the historic center for 2-3 nights and then move to a different area. Personally, I like to stay put when I travel places and just pick the location I want to be at "night", because even though I'm not a night owl, sometimes it's just nice to walk out of your hotel and go somewhere nearby without having the worry. I used to live in Coyoacan and walking from a big event in the square I walked down some deserted streets and wasn't to keen about that. So for many reason I moved to an area in Roma that I can walk anywhere including to Reforma at night. So, if you have no desire to go out at night then stay at the Circulo and enjoy the entire historic district for your entire stay. The hop on/off buses are basically right down the street from your hotel.
it's a very good way to see the entire city without getting an uber/bus/metro and they have vans that tour around the city too to different venues. When I first arrive I lived in benito juarez and would catch the metro downtown for 3 days straight to get on every hop on bus they had at that time, which was better than the ones they have now, because it would take you all the way to Tlalpan. Anyway, for 3 days I got a good tour of the city and would do it again, and I have. lol Hope this info helps you.