My personal choices:

Towns (Riverwood, Rorickstead, DarkWaters, Morthal): Expanded Towns and Villages
Falkreath: only Bring out the Dead. Love the town as is
Whiterun: Hand picked textures from Beth DLC, sixy Whiterun and Alternatives: Whiterun. For design - The Pines of Whiterun. Eariler used A Better Whiterun 1.6 which is the best city overhaul imo, but it bugs one of the houses during civil war progression. Dont use anything for outside.
Solitude: full sixy Solitude (for textures), Solitude Enhanced - new Version, Better Docks
Riften: just a very few textures from sixy Riften, Amazing Cities: Riften
Windhelm: full sixy Windhelm, Towns and Villages Enchanced - Windhelm
Markarth: Markarth Plus
Dawnstar: Dawnstar Expanded, Better Docks
Winterhold: Expanded Winterhold Destruction Ruins

Also Crowded Cities, Populated Cities, Lively inns for making them feel more alive.

My personal choice? Open Cities, hands down no questions.

Pros: The cities are freaking open! No load screen or anything, doors just open up and you freely walk in.

Cons: The performance cost. You can take a drastic hit being near or looking towards a city if your in the same cell area. Your computer better be beefy enough to take the hit to enjoy it. Also any mod that adds/alters the vanilla cities is not compatible. The creator would have to make a patch for their mod to work with OC.

I feel the rumblings of Arthmoor near by... (Runs and takes cover) lol