This is a question I never seem to like answering. If you're asking for the deck that wins the most, your answer will obviously be Kingdra and Dusknoir variants. However, you have to keep in mind that these are the decks that nearly every single player plays-- this is partially because they are very good decks, but also is a consequence of conformity. When people see that a good deck is winning several games, everyone builds the exact same deck or some variant of it, and before you know it all you're doing is trying to find counters to a single deck in your game. I think the best decks to play are the ones that your opponent hasn't already prepared to counter. Everyone knows what kingdra does and different techs to counter it. Everyone has a handful of unknown G's in their deck to shut down dusknoir/gengar/turn one machamp. However, there are some seriously strong decks that I've seen completely outplay the usual "metagame clone deck"-- decks like tangrowth, gliscor variants, GG/weaville, magnezone, and types of eeveelutions are all examples of great decks that people aren't already prepared to play against. Do they win all of the time? No, they still need to be well constructed and thoughtful decks, but they often catch the unprepared player and win through a kind of element of suprise. It might not be the answer you're looking for, but it's definitely something to think about
