Devin Booker should win performance of the year but he obviously won't even though most game value metrics rate his performance as the best individual performance of the year and it was also against the toughest opponent.
It was a great performances but I don't think it's the best of the year considering the Suns lost, by 10 points and trailed all game. That would be like Spoelstra winning coach of the year despite the Heat missing the playoffs. His is the only performance in which his team lost, which will hurt him in the voting. I'm not discrediting it, I don't think it's as impressive as some of the others from this season. For example, in Thompson's game he could have had 70 and the win if he played in the 4th quarter.
The other choices are very impressive as well, each for different reasons. In the case of Harden and Westbrook I'm sure it was difficult to choose which game was their "best performance" but Thompson's occurred first this season and I think set the bar for individual performances and I'm not sure it was passed by anyone else. He shot 63% from the field, 21-33, plus 8-14 from 3pt range, and only 10 of points came from the foul line. He had a +/- of 37 in his 29 minutes without playing in the 4th quarter.
Here are a links to the Youtube highlight videos and boxscores for each nominee, I listed them in the order I think they should finish.
Klay Thompson's 60 points through 3 quarters/29 minutes vs Indiana on December 5th
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http://www.espn.com/nba/boxscore?gameId=400899756
Harden's 53 points, 16 rebounds, and 17 assists vs the Knicks on December 31st
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http://www.espn.com/nba/boxscore?gameId=400899946
Westbrook's 57 points, 13 rebounds, and 11 points vs the Magic on March 29th
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http://www.espn.com/nba/boxscore?gameId=400900507
And Booker's 70 point game vs the Celtics.
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http://www.espn.com/nba/boxscore?gameId=400900471