Kevin Love.
That may sounds crazy as he is a multiple time All-Star, I know, but all signs point to Kev being one of the most improved players this upcoming season.
We all see in the final stages of the regular season last season and in the Boston series how he was finally starting to blend in with the Big 3 and was creating a nice groove for himself. With hopefully a close to full off-season under his belt, granted the shoulder is ready, which by all accounts will seem to be the case for the most part, Love is primed to get back to his All-Star form. The multiple conversations his had with LeBron over the off-season indicate both parties are willing to make this work and Coach Blatt will be prepared to get Love more touches on the elbow and better utilise his abilities. Also, with Kyrie out for the first portion of the season, it makes Love the second option and hence, more touches. This will also finally allow us to witness the unstoppable concept of a LBJ/Love pick and roll/pop, which will be a nightmare to defend.
Also, if the Cavs roll with Shumpert in the starting line-up instead of J.R, it will open up more shots in the starting line-up, shots that Love will be able to take and use to his advantage. Starting Mozgov also will help cover some defensive deficiencies in Love's game.
With hopefully a full bill of health to begin the season (remember his constant back injuries all of last season), Love will be back at All-Star form. He averaged 20 and 9 on 2 days rest last season, proving that when healthy and fresh last season, he was still as good as ever. His rebounding is still elite as well.