AlphaZero was indeed the best when it burst onto the scene in December 2017. Even if the DeepMind team had allowed Stockfish to run on state-of-the-art hardware, and gave Stockfish a high-quality opening book, I still believe AlphaZero would’ve beaten it over a 100-game match, although it would be by a smaller margin. AlphaZero made some moves in those games that traditional chess engines using alpha-beta pruning and other usual algorithms could never comprehend making, nor could they evaluate those moves correctly. It’s been a long time, but my recollection was that some human observers called AlphaZero’s moves “alien”.
The more important point was that this led to a complete paradigm shift in the computer chess community. People were shocked that a program which started more or less from scratch, programmed only with the bare rules of chess, after 9 hours of self-training was able to topple fifty years of computer chess development and algorithm refinement. It was like America witnessing the USSR make the first claim to space with Sputnik.
The reason you never heard any more about AlphaZero was that DeepMind/Google considered it a proof-of-concept and ceased development on it after publicizing these shocking results, but the computer chess community immediately realized that reinforcement learning with neural nets was the new best approach going forward. The community picked up where DeepMind left off with Leela Chess Zero, an open-source project that uses many of the same techniques from AlphaZero, but with computing power donated by volunteers. In addition, the Stockfish team has also adopted elements of AlphaZero, in particular the latest version of Stockfish uses an efficiently updatable neural network (NNUE) which is optimized for CPUs, whereas Leela Chess Zero uses mainly GPUs.
Nowadays, the latest versions of Leela Chess Zero and Stockfish are considered to be quite a bit stronger than AlphaZero. See these two recent Reddit threads for some more info (thread 1, thread 2). Even though AlphaZero is proprietary and Leela Chess Zero and Stockfish cannot face it in a series of games to assess the difference in strength, people have tested them against the same version of Stockfish (Stockfish 8) that AlphaZero faced back in 2017, under similar match conditions, and found that they crushed Stockfish 8 by an even larger margin than AlphaZero.