I would recommend to start your journey in Belgrade with Nikola’s walking tour. He’s professional and very willing to share his knowledge and opinions (ask him questions - from the histories to politics and food/coffee).
Nikola is a professional tour guide showing us around the main city sights and share the histories behind (with a logic). I joined another tour of Nikola’s and found it informative so I reserved for this right away. It was definitely worth of time.
Olivera is very good at finding the interesting part of each street art and explain them with context and background. I can see it not easy to collect those information so it’s valuable to join this tour and learn the insights.
Aleksandar is the best guide you can imagine. He’s super knowledgeable (not only for Skopje or Macedonia histories but even for other cultures), has a great sense of humor, and shared his opinions and observations. Definitely recommend start your journey in Skopje with Aleksandar.