Part 3 - Graphical Mods

Architecture

Hlaalu

Check out DarkElfGuy's comparison video here.

Hlaalu - Arkitektora of Vvardenfell by Tyddy

Texture replacer for Hlaalu architecture.

Notable alternatives are AnOldFriend's Hlaalu Retextured and Kel's Hi-Res Hlaalu Retex Pack.
White Suran 2 - MD Edition by Melchior Dahrk and Basswalker

Gives Suran a unique white appearance.

Includes patches for Slartibartfast's Poorly Placed Object Fix, Matthew's Suran Underworld and Vurt's Ascadian Isles II. If, for some reason, you want to use the previous version of this mod, it also includes some fixes for that.

Redoran

Check out DarkElfGuy's comparison video here.

HD Architecture Meshes by Colt17

Smoother meshes for Redoran architecture.
Redoran - Arkitektora of Vvardenfell by Tyddy

Texture replacer for Redoran architecture.

Telvanni

Check out DarkElfGuy's comparison video here.

Telvanni Exterior UV Fix by Plangkye

Fixed meshes for Telvanni architecture.
Hi-Res Telvanni Textures by Plangkye

Texture replacer for Telvanni architecture.

A notable alternative is Tyddy's Telvanni - Arkitektora of Vvardenfell, but it caused lots of errors for me.
Telvanni Mesh Improvements by Champion of Hircine

Smoother meshes for Telvanni architecture.

Imperial

Check out DarkElfGuy's comparison video here.

Improved Imperial Fort UVs by SGMonkey

Fixed meshes for Imperial Forts.
Imperial Houses and Forts Retexture - Ordo Arkitektora by Tyddy

Texture replacer for all Imperial architecture.

I use this just for the Imperial Forts - the house textures will be overwritten later.

If you want to use Tyddy's textures for the Imperial houses as well (instead of the mods suggested below), be sure to download the "Retex-Patch for AOFs Imperial Houses BumpMapped", which contains the normal maps to be used with Elmer's Bump-mapped AOFs Imperial Housing.
Imperial Housing Retextured by AnOldFriend

Texture replacer for Imperial houses.
Bump-mapped AOFs Imperial Housing with optional Vanilla-Friendly Retexture by Elmer

Adds bump-maps to AnOldFriend's retextured Imperial houses, and includes some more vanilla-friendly textures.

Requires AnOldFriend's Imperial Housing Retextured, even if using the optional vanilla-friendly retexture (recommended).

Comparison: AnOldFriend | Elmer
Ebonheart Dragon Statue Retexture by StoneFrog

Retextures the famous dragon statue in Ebonheart.

Vivec & Velothi

Check out DarkElfGuy's comparison video here.

Velothi - Arkitektora of Vvardenfell by Tyddy

Texture replacer for Velothian architecture (e.g. Vivec and tombs).

Notable alternatives are MikeandJosh's Vivec and Velothi Retexture and AnOldFriend's Vivec-Velothi Retextured, although the latter I find a bit grimy.
You Just Got Frescoed by Aeven

Texture replacer for the Velothian frescoes.

Personally I prefer these to the frescoes that come with Tyddy's Velothi - Arkitektora of Vvardenfell.

Comparison: Tyddy | Aeven

Dwemer

Dwemer Mesh Improvements by Champion of Hircine

Replaces 22 Dwemer models.
Darknut's Better Dwemer Towers by Darknut

Replaces the Dwemer towers with improved meshes while preserving the original octagonal style.
Full Dwemer Retexture by Tyddy

Texture replacer for everything Dwemer.

This mod is not limited to just the architecture, but a notable alternative to the retextured architecture is Darknut's Dwemer Ruin Textures.

Daedric

Check out DarkElfGuy's comparison video here.

Daedric Ruins - Arkitektora of Vvardenfell by Tyddy

Texture replacer for Daedric Ruins.
The Daedric Statue Replacer Pack by Astion

Texture replacer for Daedric statues.

Comes in 3 styles: normal, marble and gold.

Requires cleaning.

Strongholds

Stronghold Retexture by Flash3113

Texture replacer for Dunmer Strongholds.
Propylon Pillar Retexture (PPR) by Alaisiagae

Texture replacer for the Propylon Pillars found in Dunmer Strongholds.

Includes optional Propylon Index retexture.

Sewers

Sewers - Arkitektora of Vvardenfell by Tyddy

Texture replacer for sewers / underworks.

Shacks & Docks

Shacks and Docks Retextured by AnOldFriend

Texture replacer for shacks and docks.

Notable alternatives are Tyddy's Shacks Docks and Ships - Arkitektora of Vvardenfell and Earth Wyrm's Shacky Shacks(!), but personally I found some of the retextures in those mods to look a little out of place (e.g. the bridge in Seyda Neen).

Mournhold

The Mourning of Bamz-Amschend by Aeven

Texture replacer for the ruins under Mournhold.
The Clockwork City by Aeven

Texture replacer for Sotha Sil's Clockwork City.
Green Marble Mournhold by Plangkye

Texture replacer for the Mournhold architecture.

Part 3 - Graphical Mods

Architecture

Hlaalu

Hlaalu - Arkitektora of Vvardenfell by Tyddy

White Suran 2 - MD Edition by Melchior Dahrk and Basswalker

Redoran

HD Architecture Meshes by Colt17

Redoran - Arkitektora of Vvardenfell by Tyddy

Telvanni

Telvanni Exterior UV Fix by Plangkye

Hi-Res Telvanni Textures by Plangkye

Telvanni Mesh Improvements by Champion of Hircine

Imperial

Improved Imperial Fort UVs by SGMonkey

Imperial Houses and Forts Retexture - Ordo Arkitektora by Tyddy

Imperial Housing Retextured by AnOldFriend

Bump-mapped AOFs Imperial Housing with optional Vanilla-Friendly Retexture by Elmer

Ebonheart Dragon Statue Retexture by StoneFrog

Vivec & Velothi

Velothi - Arkitektora of Vvardenfell by Tyddy

You Just Got Frescoed by Aeven

Dwemer

Dwemer Mesh Improvements by Champion of Hircine

Darknut's Better Dwemer Towers by Darknut

Full Dwemer Retexture by Tyddy

Daedric

Daedric Ruins - Arkitektora of Vvardenfell by Tyddy

The Daedric Statue Replacer Pack by Astion

Strongholds

Stronghold Retexture by Flash3113

Propylon Pillar Retexture (PPR) by Alaisiagae

Sewers

Sewers - Arkitektora of Vvardenfell by Tyddy

Shacks & Docks

Shacks and Docks Retextured by AnOldFriend

Mournhold

The Mourning of Bamz-Amschend by Aeven

The Clockwork City by Aeven

Green Marble Mournhold by Plangkye