The city of Thessaloniki is not nice. The Byzantine churches are.
Most of the churches in the lower town are squeezed between ugly modern buildings. I liked the upper town better. Some of the old Ottoman architecture has been preserved in the narrow streets and steep staircases. The frescoes there are among the most beautiful in Thessaloniki, but the monasteries are rather inconspicuous from the outside. I walked past the entrance to the Latomou monastery twice before I finally found it.
Assif wrote in his review that it is impossible to find out the opening hours of the churches. This has not changed until today. You can‘t rely on the information provided on the Thessaloniki tourism website or on Google Maps. Even the info boards at the entrance to the churches are not fully reliable. I stood in front of the closed door to the Church of the Saviour three times, although the information board said: open daily from 7am to 3pm. But I only missed this one and the Byzantine Bath (closed on weekends), I could visit all the other monuments from the inside.

I visited Thessaloniki on a weekend in September 2023. On Saturday afternoon I found four churches open, either after a wedding (Hagios Demetrios, Vlatades Monastery) or after evening mass (Agia Ekaterini, Prophet Elias). But all churches were open on Sunday (with the exception mentioned above), from the end of morning mass around 10am until late afternoon. Photography was allowed in all monuments.

My highlights:
Most beautiful frescoes: definitely in Nikolaos Orfanos (lower photo). It is a relatively small church but almost the entire interior is covered with frescoes.
Best mosaics: Hagios Demetrios and the apsis of Latomou Monastery. From Latomou you also have a nice view of the city and the sea.
Most beautiful exterior: Agios Aposteles (upper photo). The decorations of the brickwork are striking. The church is located at the eastern end of the city centre, close to the city wall, in a quiet neighbourhood.
Largest church: Hagia Sophia. The mosaic in the apsis is remarkable. But I was most fascinated by the chandelier with its countless griffins.
Most impressive architecture: Rotunda. It has nice mosaics but it is the architecture of the domed building that impresses the most. In particular because it is the oldest of the inscribed monuments, dating from the 4th century.

The Rotunda is a museum and therefore has regular opening hours. The Old City Wall is also part of the WHS, large parts of which are still intact. The best preserved sections are in the upper town: the Heptapyrgion citadel, the Trigonion tower and the section near the Vlatades Monastery. I wouldn't say that it's a must-see, but as already mentioned, the upper town is the best part of Thessaloniki, very different from the noisy and busy lower town. If you have enough time, it's worth wandering around there.
I completed my visit to Thessaloniki in the Museum of Byzantine Culture. The permanent exhibitions are magnificent and cover the entire Byzantine period. The last room describes the steps from the excavation of an object to its presentation in the museum.

When visiting Thessaloniki, comparisons with Ravenna come to mind, Nan mentioned this in his review. I was also more impressed by the mosaics in Ravenna. And many of the frescoes and mosaics in Thessaloniki are not in good condition, often only fragments are preserved. However, the abundance of monuments and works of art is remarkable. The entire Byzantine era is represented, from the 4th to the 14th century.