Hydreigon.
After I dropped pokemon for a few years, I picked it back up after Sword and Shield came out. I then decided to play through all the pokemon games I had missed over the pass few years, one of which was Black 2. During my Black 2 playthrough, I encountered a shiny deino in victory road. Up until that point I hadn’t seen or even thought of Deino for years, so seeing a shiny one was exhilarating. I caught it, named it Yokoshima (Japanese for evil) and immediately trained it up to be a hydreigon. I then used that Hydreigon to sweep the league. Now that hydreigon is viable again in SV, I have used it on almost every team I built.