Best World for Medieval save

There's a section of Windenburg that's more on the modern side even if you replace the surrounding lots. Henford on Bagley has rolling hillsides and overall is more fitting for medieval in my opinion... but I don't play historical saves.

Before Cottage Living I think those who played historical games loved Windenburg, but now Hendford I think is really the best. I have yet to play the decades challenge, but I have some historically dressed and living sims in both and well as my elves. Glimmerbrook is also possible.

The Werwolf world.

I agree with the Henford Vote. Much as I love Windenburg, it does have the modern area that kind of takes the timeless immersion away. Henford's town is more believable in that way. And it would probably be generally easier to use for that gameplay theme.

But you could still use Windenburg, I think. In my game I've replaced the lots on the modern side for older looking buildings and mostly try to play in the older areas anyway. Windenburg is the largest Sim world, it has the most lots (and the ruins too!) so eliminating the modern section from gameplay still leaves a normal sized world to play with if you really wanted to use it.

I'd also say Forgotten Hollow and Glimmerbrook, but they are quite small so there wouldn't be much to work with unless you used them as extension villages to visit. Tartosa maybe would work as well, though I don't have that world, so I'm not quite sure.

I think Henford on Bagley is the best. There are a few deco cars in the world but for the rest it is very good for historical saves.
Windenburg is another good option. 3 of the 4 neighbourhoods (island, old town and country side) have surroundings that feel fitting for medieval saves. Especially the island is useful because it has no decorative buildings or modern elements.
Another good option is Glimmerbrook. It is remote without any decorative buildings to determine the era.

If you are asking because you need to limit it to only one world (because you can only buy one EP, or are doing a challenge with world limitations, etc.), then I would probably go with Henford.

Otherwise, if you have no real reason to limit it to just one world, I would suggest putting your primary household in either Henford, or the rural or island sections of Windenburg, but then designate some combination of Henford, Windenburg, Forgotten Hollow, and/or Tartosa as areas where you can have "neighboring villages" with non-residential lots (shops, churches, parks, etc.) that you can utilize and other households to befriend or romance.

I like Windenburg because of the club aspect. The Knights of the Hedges give me medieval vibes so that’s how I would incorporate those together.

For medieval play, my vote goes to Windenburg, Henford, Forgotten Hollow AND the uninhabited Island on Sulani!

The Crumbling Isle and the northern district in Windenburg work great for huge castles and taverns. I avoid the modern district altogether. Most of my Lords and Ladies live in Windenburg, whereas in Henford is where all me lovely commoners and some bog witches reside. I have pirates and medieval merfolk in (googles) Mua Pel'am in Sulani and then there's Forgotten Hollow where the Hunters live. Hunters as in, pale looking Sims with fangs and also those that are extremely hairy with claws.

Seeing cars and jetskis drive by and Nancy Landgraab and the Bjorgensens in modern clothes would definitely break the immersion for me. There are a few secluded lots here and there, maybe if you don't mind having your medieval lots spread in several different neighbourhoods, it could work. That being said, Old town in Windenburg has beautiful bell chimes sound effects, I think that would be a nice setting and it needs few adjustments if you want to play a medieval city. Henford is a good choice too, especially in the countryside area.

Also, wanted to add that my answer was based on my interpretation that you are looking for realistic/historical medieval settings. If you are looking for a quasi-medieval world in a fantasy save, the answer would be slightly different.

E.g., one of these days, if I can tear myself away from my main save, I think it would be fun to do an Elder Scrolls save, in which case I would do something similar to what I described above, but add parts of Oasis Spring (Alik'r) and Sulani (Stros M'kai), etc.