If I had to pick? Maybe England/ Scotland, Estonia/ Scandinavian countries, and/or Finland? Germany? I think maybe save that for a tour though. ETA- Is there a good base to work out of in Germany?
There are several good places to base in Germany. And Germany has the advantage of having excellent public transportation. I spent 4 weeks there in 2022, and used the train system for almost all transport. The only time I rented a car when I was staying in a small town that my ancestors came from. The nearest train station was 2.5 miles away, so I opted to rent a car for the 4 nights I was there.
There are a lot of castles in the Rhine area, and that would be a good place to base in. You could explore the northeast area while there, including places like Cologne, Aachen, and Trier. And, of course, explore the Rhine river and take boat cruises down the river.
I also really loved the Black Forest. When I was there, I stayed in Gengenbach, which was a really lovely village. And that got me to a lot of places in that area. You could even take day trips over into Alsace in France, which is very close. For example, Strasbourg is only 40 minutes away by train.