Ok, so here's a controversial opinion: I don't think Andy Murray should win it! He won it last year and two year's before that. Yes, what he has achieved is incredible, but he has that opportunity every year and throughout the year. I think someone in a sport where the Olympics is the pinnacle, should win. My personal choice would be Mo Farrah. He has won double gold in the last 4 global championships, including 2 Olympics. This is unparalleled and in my opinion he is very underrated - never making the top 3 at SPOTY.
As a swimming parent I would also support Adam Peaty, who doesn't seem to be anywhere in the running. The margin of victory in his 100m Breaststroke was 1.5 seconds, in a sport where hundredths often divide the places! His winning margin was the equivalent of Usain Bolt's win in the 200m when he set the world record. I know the hours of dedication that would have been needed to produce that result. Unfortunately a very underrated sport.