Most visited: Whiterun but only because I have to, I actually don't like it
Least visited: Falkreath but I actually don't mind it I just don't go there because there's nothing to do except ill met by moonlight
Favorite: Solitude love it everything is just so vibrant and it has the only clothing shop in the whole game
Least favorite: Morthal its like a giant half doughnut I just don't like the layout but I am forced to go there and become thane simply for the best house in Hearthfire
Most and least visited probably depends on whether you're using fast travel or not. If you are not, then Whiterun's central location and ease to find at night makes it the city I travel to the most.
Falkreath's where I travel to the least. It's out of the way and there are few reasons to travel there unless you're a member of the dark brotherhood. Even Winterhold has a few key quests involved there (like The Black Star), and the Mage's Guild all but requires that you stroll through town on the way.
I rarely go to Mortal, not sure why I just don't...
No most visited depends on my character... Right now Windhelm as that is where I am based... My other charater has no home I am playing Markus' build the Archer... But, I still tend to go to Windhelm to do my trading and whatnot... Although I like Riften too it has a different set of advantages
My most visited is Whiterun, mainly because the character I played as the longest both owned Breezehome and was working for the Companions, so I had to make many trips back there from wherever I was. But even on my other characters, I have spent the most time there. I think it's partly because it's the most centered city in the game as well as the fact that many useful things happen to be there.
Myu least visited city, that's a though one. But I would have to say probably Morthal, mostly because I don't get a lot of reasons to go there so I just simply skip passing through unless I absolutley need to.