In terms of looks and ambience, Whiterun is probably the most "pleasant" place. However, it seems like 70% of the game's most mind-numbingly annoying NPCs are there - Nazeem, Braith, Belethor, Olfrid Battle-Born, Sigurd, Mikael, Mila Valentia, Heimskr, Jarl Balgruuf's kids, GODS MAKE IT STOP. Believe me, Whiterun gets a lot more "pleasant" when you murder everyone that previously resided in there.
So I'd go with Solitude myself. There are annoying NPCs there too (Taarie and Endaarie, anyone? Not to mention the damn kid who says he thought "adventurers were supposed to look tough"... damn little brat), but a lot less, and since the city's bigger, it's easier to avoid them.
As for the others:
-Riften and Windhelm look nice, but their atmosphere is too cold and hostile for me to consider them "pleasant."
-Winterhold feels... dead, pretty much. Dead and empty.
-Falkreath would be pleasant if it weren't for the huge graveyard, which makes it feel like the Grim Reaper is just around the corner, casually drinking mead.
-Morthal is okay. Would be pleasant if it weren't for the burnt-down house, the ghosts and the vampires. I feel like their shadows are always looming over the town, even after you finish the quest, especially with the necromancer who lives there. I know he's harmless, but still... Plus, the Jarl is kind of a crazy old "seer", correct me if I'm wrong? It all makes Morthal seem creepier than it should, IMO.
-Dawnstar is pleasant, but I don't really like its layout. I'm too dumb for it, so it confuses the hell out of me and I always end up getting lost in the town. Yes, I am serious. What can I do? *shrug*
-One little place I always liked was Ivarstead.
-Markarth looks nice, and feels nice as well until you learn about its background; after that, it starts to feel colder and more hostile. However, I still find it a lot more pleasant than both Riften and Windhelm.
-Raven Rock, for me, has the same, for lack of a better word, "issue" as Riften, Windhelm and Markarth: it is too hostile. I can't really consider it pleasant at all.
-Riverwood is pretty nice as well. There is Ralof's annoying bratty nephew, Frodnar, but he can be ignored as well. At least he's ONE annoying NPC as opposed to 10+.
I'm not sure if I'm forgetting anything, but for what I can recall, there it is.