Antonov was truly a marvel. Here are my favorite HL2 beta concept arts from him. I really like that old citadel design, it looks so simple at a first glance, yet when you look at it deeply, it's so intricate. On the other pictures, you can see metrocops rushing, in high alert, they are coming from, a big door, really makes you wonder, what is behind that door, where are they going? And my favorite is the last image, human architecture torn apart, red sky from so many portals opening up. You can see the smoke coming from a smaller building, and you can see really well, how the 7-Hour War is going, how powerful the Combine really is, and how the biggest buildings that humanity made, being torn apart in seconds. Rest In Peace.
I prefer the content of Half-Life 2 Retail and not the Beta because it has its own identity with scenarios that the Source Engine was known for (I still liked the gameplay and scenarios of Raising the Bar - Redux). One of the best skins I loved was the Combine APC, I think this look is really cool and the APC was made by Antonov himself. Show.
Third one is by Dhabih Eng, and fourth one has no official credit.