This is for me actually a harder question to answer, first instinct I want to say one of RJ, Bacot, or Cadeau. All 3 are extremely important yet we do have nice depth at the positions those 3 play. I actually feel the real difference making is production from the 3 and the 4 spots. See, if we get production solid from those 2 spots it makes it really hard to double off them on to Bacot, RJ, or EC. When teams do double off our guy at those 2 spots then the result should be wide open looks.
So 1 key individual player, at the 3 I am looking at Ingram/Ryan/Paxon. Looking at Withers, Jalen, High at the 4. To be a dominant team I would maybe look to Jalen just because I think once he starts to "get it" he could be a lotty level guy. Ryan is cash money and Ingram is a later day version of Danny Green, that kind of do it all guy that helps you win in many ways that do not show up on a stat sheet. Withers is an X factor, really athletic, runs the court, has played and can defend multiple positions and may be primed for a step up season.
By a razor thin margin I am going to go with Ingram, he just gives you everything you need. He scores, he defends, he runs the court, seems like he is always around the ball, he can play multiple positions all at a high level, makes the extra pass and is a really good passer, solid basketball IQ, can get his own shot, rebounds and can start the break on his own, nice high motor/high energy. He is what seems like the perfect compliment that allows RJ/Bacot/Cadeau to operate freely. He could in my mind anyway be the real key for us but I say that with a razor thin margin. Every great team needs that glue guy that brings it all together, guys like Theo/Luke/Matty Doh/Danny, guys that may not stuff the stat sheet every game but guys you simply have to have in order to win big, I think Ingram may fit that mold.