Before we discuss who has the best forehand or backhand, we should first establish some consensus what constitute a great forehand or backhand.
There are many attributes to constitute a great forehand and backhand, I'll list some essentials of them here, you can add more:
Consistency, self explanatory
Power, penetrating opponents' defense, good reference is against choppers
Pace, how fast it can be executed repeatedly
Versatility, ability to change the placement, spin and pace
Usage, how often you use the shot to win points, good for these forehand or backhand dominant players
Winning rate by the shot, important if you win the majority of points by your favorite shot.
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Other less essentials:
Mechanically sound, good example is Ma Long's forehand and Aruna's
Overall winning percentage, you have to be at the top 20 or 30 in your sport to be qualified in the discussion
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Based on the essential listed above, I'll say Ma Long has the best forehand as he has top score on all essentials except power.
Also based on the essentials above, one player, Xu Xin, many folks ignored here also is in the top five forehand. Xu Xin's forehand lacks the pace as he often loops far from the table, but he excels in all other departments.