WARNING: Town mods may lower your frames, this is the down side to town mods. Using these mods will more than likely cause a drop in frames.
First off, the big overhaul:
JK's Skyrim: https://www.nexusmods.com/skyrimspecialedition/mods/6289
This mod will overhaul all the towns and cities in Skyrim. Mostly adding clutter and making the cities feel lived in, but the smaller towns will get a few new buildings, mostly shops, and NPCs.
Mods that add to already existing towns and cities:
Sky City - Markarth Rising: https://www.nexusmods.com/skyrimspecialedition/mods/22482
A port of the LE mod, this mod overhauls the exterior and interior of Markarth, building up into the mountain side. Making the city feel taller and bigger than what Bethesda gave us. Best part is that it works with JKs without any patches needed.
Windhelm Lighthouse SSE: https://www.nexusmods.com/skyrimspecialedition/mods/22315
Not really a town mod, nor is it a must have. It's just a nice little immersion building mod.
Cities of the North - Dawnstar: https://www.nexusmods.com/skyrimspecialedition/mods/28952
A new set of unique buildings for the capital of the Pale. JKs Skyrim compatible!
Cities of the North - Morthal: https://www.nexusmods.com/skyrimspecialedition/mods/34168
A new set of unique buildings for the capital of Hjaalmarch. JKs Skyrim compatible!
Cities of the North - Winterhold: https://www.nexusmods.com/skyrimspecialedition/mods/40088
A new set of unique buildings for the capital of Winterhold, JKs Skyrim compatible!
Cities of the North - Falkreath: https://www.nexusmods.com/skyrimspecialedition/mods/56731
A new set of unique buildings for the capital of Falkreath, JKs Skyrim compatible!
Enhanced Solitude SSE: https://www.nexusmods.com/skyrimspecialedition/mods/27816
Now this is the big one, this mod overhauls all of Solitude to make it into the proper capitol of Skyrim. This mod combines Solitude Reborn, Books of Skyrim, Solitude Bathhouse, and many small shops into one big ESP. Best of all it works with JKs Skyrim (With a proper patch, ofcourse.). This mod also adds an exterior pathway up to the city, giving it a more important feel to the city.
Solitude Docks Updated: https://www.nexusmods.com/skyrimspecialedition/mods/33777
The best Solitude Docks mod.
Capital Windhelm Expansion: https://www.nexusmods.com/skyrimspecialedition/mods/42990
A massive overhaul of Windhelm, changing the layout and look of the city. It also adds new houses, NPCs and areas to explore. Best part is it has a JK Skyrim Patch.
Capital Whiterun Expansion: https://www.nexusmods.com/skyrimspecialedition/mods/37982
Just like the Windhelm mod this mod overhauls Whiterun, giving it more shops, buildings and NPCs. It also adds more housing districts. It's a must have mod. Oh and it even has a JK patch.
The Great Town of Shor's Stone SSE: https://www.nexusmods.com/skyrimspecialedition/mods/35977
Overhauls the little mining settlement into a somewhat proper town. Has an Inn now!
Mods that add new towns:
Adds a very comfy town in the Rift.
Granite Hill Village: https://www.nexusmods.com/skyrimspecialedition/mods/58335
Adds a comfy little village between Fort Sunguard, Bloated Man's Grotto and Falkreath Hold. Small but lively.
Reich Corigate: https://www.nexusmods.com/skyrimspecialedition/mods/58214
Another small, but comfy, village. This time located in the Rift by Treva's Watch.
Yet another small and comfy village located between Dustman's Cairn and Silent Moons Camp.
Amber Guard: https://www.nexusmods.com/skyrimspecialedition/mods/55564
This mod adds a little village near Karthwasten and Broken Tower Redoubt.
Dunpar Wall: https://www.nexusmods.com/skyrimspecialedition/mods/60728
Adds a village at the crossing between Fort Dunstad, Fort Fellhammer and Dawnstar.
Quest mods that add towns:
Moon and Star: https://www.nexusmods.com/skyrimspecialedition/mods/4301
Adds a small dunmer village that has its own quests and a shop.
Helgen Reborn: https://www.nexusmods.com/skyrimspecialedition/mods/5673