I was just about to mention Belgrade, imo it has the best brutalist buildings outside of the former Soviet Union countries, that's (also) why it is on my to-visit-soon list.
Might be helpful for people interested in this, an acquaintance of mine recently started working on a project for an online archive of socialist modernist concrete-based (so not only brutalism in strict sense) architecture (contains photos, info, publications, art projects inspired by the subject, etc.): https://belgradesocialmodernism.com/
That's why I've said "outside of the former Soviet Union", inside the former Soviet Union there are cities which can compete with Belgrade on the brutalist front, from what I was able to see from IG Sankt Petersburg and Kyiv are quite interesting on that front. To say nothing of the brutalist Soviet bus stations which deserve an architectural category/style all for themselves [1]