If “Best Player” means “contributes the most to winning basketball games”, then it’s Al Horford.
Absolutely correct. Al would also be the single most difficult player to replace if we lost one of the starting 5 due to injury. The flip side is that this team's ceiling is limited if someone else doesn't emerge as the most impact player when the playoffs arrive.
Irving and Tatum are obviously the most important when it comes to generating offense in tight games against elite competition.
It's fair to argue that Hayward would be the best player if his game recovers completely by the end of the season. Similar to Horford, he's the only player who can consistently provide positive contributions in every aspect of the game.