...I want to see the various architectural styles of High Rock and Hammerfell at their best? For example, Ald'ruhn is the best display of Redoran-style architecture on Vvardenfell, Balmora for Hlaalu architecture, and so on. What cities and towns in this game are, like those examples, the "go-to" places to see all the different styles of architecture? The number of quests or people they contain doesn't matter to me, at the moment I'm only interested in the buildings and the layouts of the towns.

The three main capitals would be the best choices, for the sole reason that they're the only cities that are fully hand-made and not randomly generated.

So,

Daggerfall in Daggerfall
Wayrest in Wayrest
Sentinel in Sentinel

Daggerfall and Wayrest will give you the feel of the architecture of High Rock's coastal cities and Sentinel for Hammerfell.

But I'll add Wrothgaria in the Wrothgarian Mountains province for the mountain architecture. The citizen's clothing is different too.

Happy exploration!

I found the orsinium stronghold to be quite interesting as well as the Direnni Tower.

Other than what Cliffworms said, a good portion of the architecture is recycled.

Castle Wayrest looks rather contemporary in design. The architecture looks incredibly modern in comparison to that of Cyrodiil.

Crap, you're right, I forgot those two. Awesome architecture.

Wayrest indeed has the architecture from 18th and 19th century european castles. But it looks so great at night.

When I first played DF Wayrest was always my go-to city, I loved the architecture and the fact that I was never assaulted by ghosts. But back then I never played the MQ, so once I ventured into that I never really liked Wayrest the same again.

But anyways, there is also the swamp architecture too. None of the countries are particularly noteworthy, though Lainlyn was always my go-to country for thievery and general illegal activities.

Where would I find some of the swamp architecture?

Totambu and other regions surrounding it. On the south east of the map. (But not the dragontail mountains.)

Obviously. The Wayrest Kid's full dialogue is actually "Ahhhhhhoooooooooooo, these Baroque and Neo-Classical fixtures strike me as discordant with the High Gothic and Norman forms typically observed in other regions of the Bay. This is sure to complicate my dissertation on modern architectural philosophies." This was removed due to the space constraints of 90s PCs.

Fun facts aside, I always thought Wayrest's palace was very in keeping with its role in the story. It's the newer, upstart city on the Bay, so its architecture is going for a kind of "new-money" vibe. While it doesn't have the history of Daggerfall and Sentinel, it has the wealth to blind them with luxury, making the Daggerfall palace look like a crude relic by comparison. Also, the Wayrest royal family happens to be the most brutal and underhanded in the Bay, so it's a nice contrast that their palace and gardens seem so modern and civilized.