Sensibly, the stat that measures production with a high correlation to winning games is known as Wins Produced (WP). Additionally, all the basic stats (points, rebounds, assists etc) are listed on a per 48 minute basis. That way you aren't docking Kenneth Faried for playing rookie minutes. George Karl already holds him back enough as is.
In order to see who had the biggest improvement in WP, I listed everyone in the NBA this year, along with their primary stats (excluding rookies and anyone else that didn't play last year) and then looked up their WP from last year. A quick sort by the difference yields the following list, which includes all the names previously mentioned.
A quick bit of detail for comparability: a WP/48 of .100 is an average player, .200 is a star and .300 is Lebron, Chris Paul or Tyson Chandler (Yes Tchan is that good - check out the Dallas Mavs before and after).
It is rare that a player makes the jump to elite status as James Harden has this year. Although it is surely not easy to do, I do not like the idea of awarding a player who went from a liability to the team to something of a non-factor to nearly average (Avery Bradley, Byron Mullen, Roddy). Nikola Pekovic has one of the biggest year-over-year improvements ever, but misses on games played. Jerebko, Hibbert and Cunningham are all making the move towards star player production, but Ersan Ilyasova has a similar amount of improvment, but sits at a much higher level.
One last note. Look at the improvement LBJ made from last year. He is the 23rd most improved player in the league, and he was already top 5 in terms of Wins Produced! Anyone who hasn't noticed this is blind or in denial about the level of talent he possesses.
Can we just fast forward to a Heat-Thunder finals already? It would be a Wins Produced overload with the best shooting guards (Wade and Harden) and the best forwards (Lebron and Durant) in the league for likely 6-7 games. Make this happen!