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I spent 3 nights in Oslo and 2 nights in Bergen. I liked Bergen more than Oslo, but Olso has more "turnstyle attractions" like museums. I could have cut a day from Oslo and not missed it too much; I could have added a day to Bergen just to walk around as it was so pretty. However, I was there on two consecutive days with no rain, statistically a very rare occurrence!

As always, everyone will do things differently, and part of the answer depends on how much time you have for the total trip. I preferred both Stockholm and Bergen to Oslo; I haven't been to Copenhagen in many years and haven't been to Tallinn at all, so I can't comment on them.

I had a very similar experience to Harold's. I spent 4 nights in Oslo (but arrived at midnight and left on NiN at 6:30am), giving me three full days. One of those days I took a day trip to Fredrikstad, so with two days plus one additional evening, I felt I had time to see everything I wanted to see, including most of the attractions listed at https://www.ricksteves.com/europe/norway/oslo. I missed the ski jump and a few of the smaller museums, but I did visit the Astrup Fearnley Museum, which I highly recommend if you like modern art (and by modern I mean very modern).

I also had two nights in Bergen and enjoyed lots of sunshine. I loved Bergen but I had plenty of time to wander and explore, did an excursion to Troldhaugen, and rode the Fløibanen to see the view and hike around a little bit. I don't know what I would have done with another day there.

I've also been to Stockholm, Copenhagen, and Tallinn (also Helsinki). You will love all these places!