Although I have a fondness for modern architecture, I was not very familiar with Álvaro Siza's work when it was included in Portugal's revised T-list in 2017. His style is often classified as Critical Regionalism and is characterised by clear forms, simplicity and the restriction to the essential elements. Alvar Aalto (on the Finnish T-list) also represents this style. The list of Siza's works is extensive and includes many private homes, social housing complexes, residential areas and public buildings such as banks or swimming pools. Later in his career, he also designed museums or Expo pavilions and other representative buildings. All these buildings are still used for their original purpose. Therefore, most of them are not open to the public, or only partially during regular opening hours.
When I was preparing for my trip to Portugal in May 2018, I spent some time selecting the Siza buildings that would be worth visiting. The Unesco entry doesn't help. It is more a hymn of praise to Siza and his impact than a detailed description of his buildings. Only a few are mentioned, but it is obviously not yet decided which sites will be included in a nomination. Meltwaterfalls compiled a list of sites and coordinates here on the forum. That was very useful, so a belated thanks to him.

Álvaro Siza's buildings are spread all over Portugal, many of them are in Oporto and its surroundings. It is therefore easy to include some of these sites in your itinerary, especially if you are travelling by car.
A few comments on the eleven buildings that I visited on my trip.

Siza became known for two buildings in his hometown of Matosinhos, the Boa Nova Tea House and the Piscinas de Maré, not far from each other directly on the Atlantic coast. Both are a perfect blend with the rocky landscape in which they are built. Unfortunately, the Tea House was closed when I visited; it is today a first-class restaurant open only in the evenings. And the tide pools were not yet filled with water in mid-May. But the area was accessible and you could see how the pools are integrated into the landscape, bordered by man-made concrete walls and by the natural rock formation. I would have loved to take a refreshing bath in this oceanside setting with an unobstructed view of the sea.

In Oporto, I visited four sites. The Serralves Museum of Contemporary Art is located in the northwest of the city. The collection is worth seeing, the architecture is rather unobtrusive, very functional, the art is in the focus and the harmonious integration of the building into the surrounding park.
The Porto School of Architecture (photo) was one of my favourites. It consists of several white pavilions, the most striking are the four towers facing the Douro River. I visited the site on a weekday in the lecture period, so I was also able to take a look inside some of the buildings.
Shortly after the Carnation Revolution, Siza designed his first social housing estate, Bouça II in Porto, a neighbourhood consisting of four rows of multi-storey terraced houses. However, the complex was only completed in 2006, after an interruption of three decades. It is not far from the city centre, right next to the Lapa metro station.
And finally, the easiest tick in Oporto, the São Bento metro station, within the core zone of the WHS, under the square in front of the historic train station.

Marco de Canaveses was a short detour on the way from Guimarães to the Douro. Here is the only religious building on this list, the Igreja de Santa Maria. The church is a solid whitewashed cube with two projecting blocks at the front that function as bell towers. Only the apse is formed by two concave curves. A building typical of Siza's style.
The Adega Mayor Winery is located about 25 kilometres north of Elvas. It is an elongated building on a hill overlooking the vineyards. It looked almost a bit too picturesque: the white walls of the winery with the green of the surroundings and the bright blue sky. There is a public wine tasting area and maybe also guided tours to the production facilities, but it was already closed when I arrived there in the evening.
The Quinta da Malagueira neighbourhood, located next to the historic centre of Evora, is another social housing project. It consists of two-storey terraced houses with courtyards, and has a total of 1200 residential units. In the 1970s, this was certainly an innovative project, in the tradition of the housing estates of Berlin or Amsterdam, but for today's visitor it is not outstanding.

Siza is probably best known in Portugal for the rebuilding project of the Chiado district in Lisbon, parts of which were destroyed in a fire in 1988. It is the area right next to the famous Elevador de Santa Justa. Every visitor to Lisbon comes there but probably pays little attention to the renewed pedestrian walkways and staircases around the ruins of the Convento do Carmo.
And finally, the Pavilhão de Portugal, built for Expo'98 as the pavilion of the host nation. The most striking feature, however, is the open space between the two parts of the building. It is spanned by a concrete roof that curves down in the middle (shown in the intro photo). A daring construction and probably the most iconic of Siza's buildings.

I very much enjoyed exploring the architecture of Alvaro Siza. These buildings were welcome spots of modern architecture among all the historic WHS on my Portugal trip. My favourite sites were Piscinas de Maré, Porto School of Architecture, the Church in Marco de Canaveses and the Expo-Pavillon in Lisbon.

But should Siza's architecture be a World Heritage Site?
His style can be seen as a successor but also as a counter-movement to the International Style (Bauhaus, Le Corbusier, Niemeyer). The latter is already well represented on the WH list. So, from a chronological point of view, it makes sense to now focus on the second half of the 20th century.
But it is difficult to assess Siza's influence in comparison to other contemporary architects. There are no exceptional or iconic buildings in Siza’s work, which is also a consequence of his architectural approach. An inscription of a series of Álvaro Siza buildings would place him on the same level as Le Corbusier and Frank Lloyd Wright. In my opinion, this is not justified. Thus, a thumbs down from me.
Nevertheless, I can highly recommend to include some of these sites in your itinerary when you visit Portugal. At least for those who have a fondness for modern architecture.