With my newest toon, level 14, I have yet to see:

Windhelm
Riften
Falkreath
Morthal
Markarth
Dawnstar
Solitude

Believe it or not. I've seen them (with the exception of Morthal and Falkreath) with other toons, but this path I'm on is making a rut in the map....

160 hours in, I haven't been to Markarth yet.

I think there is only one town I haven't been to yet, Falkreath. One day I'll make it down there, once I find the Kuda Bow.

Markarth is a beast to navigate.

I've got over 60 hours on two characters & I've yet to visit Falkreath or Morthal

i've seen them all but solitude was teh last one, only went there for the final storm cloak mission, and i wouldn't have ever went to riften if not for the main quest on my first character, actually i probably haven't been back since that quest

I only know Markarth, Riften, Winterhold and Whiterun well. I have not been to Falkreath in any of my playthroughs yet. I have barely seen much of Morthal, Dawnstar, Windhelm and Solitude yet.

At aprox. 70 hrs of play, I've been only to Whiterun, Falkreath and Markarth.

I like to do everything on a city/hold once I arrive there, so I avoid to have different quests from multiple cities/holds at the same time, and geez, the forsworn at The Reach/Markarth keep you well busy

It will be accounted in history that it was the Dragonborn that eradicated the forsworn from existence.

Hehe yes i have been to every city/village.
Took a while for me.

I've seen them all... and Markarth impressed me the most.


But to answer your question in another way, the cities that I haven't seen yet are:

Alinor - Summerset Isle
Falinesti - Valenwood
Sentinel - Hammerfell
Senchal - Elsweyr
Blackrose - Black Marsh

Maybe one day I will see one (if not all) of them