which cites/ areas in germany speak "Hochdeutsch" ?
The short answer is: Nowhere really. Here's the longer answer: Hochdeutsch is, in some ways, a constructed language and if you get two people from different regions of Germany, they will hardly ever agree what even is Standard German (to give an example, I learned only a few months ago that the word for carrot that I use, is used only in a very small region of Germany and most Germans use different words). Linguistically speaking, it takes its main influences from Upper (Southern) German dialects, but you wouldn't know it speaking to people there. It is often said that the cleanest Standard German is spoken in the area of Hannover, but it's not a Hannoveranian dialect. The original dialects of that region are simply extinct. You can for the most part hear the best Standard German in Northern Germany, but once again, this is mainly because the actual Lower German dialects aren't widely spoken anymore. The ability to speak Standard German used to correlate more with level of education, as it was the language of choice used in education (my Mum likes to joke that when she started school, she had to learn Standard German as her first foreign language). These days, however, with people travelling more and access to TV and movies being very wide-spread, most people grow up with more Standard German, especially in bigger cities, and are able to speak decent Standard German. I do not know anybody my age or younger who is only able to speak a dialect (but I'm also from a medium-sized city).
my Mum likes to joke that when she started school, she had to learn Standard German as her first foreign language
Heh, I say that too. :D My mom comes from a small Swabian village, my dad is French, there was no one who could teach me to speak Hochdeutsch until I started school.
The best Hochdeutsch is spoken in the Hannover region, but it's what everyone leanrs and uses in school as well. I come from a region where we actually have the saying that we can do everything except speak in Hochdeutsch ("Mir kennet ells außer Hochdeutsch") and I can still speak Hochdeutsch without an accent if I want to in addition to Swabian, but I also grew up in a city. In cities you'll definitely find more people who have no accents than in smaller towns.