It took me quite a while to come up with this. I don't like to choose the favorite. Any of the following could be on my favorite: Jan-Ove Waldner, Jorgen Persson, Saive, Samsonov, Timo Boll, Gatien, Maze, Mizutani, Wang Liqin, Zhang Jike, Ma Long, Xu Xin, Chen Qi, Ryu Seung Min, Oh, ...
My favorite player is Ma Lin. Like Waldner, he is an artist in this sport. He can spot his opponent's weakness and make them bleed quickly. He expresses himself through match and shows the world that table tennis can be like that. Table tennis is much more entertaining and inspiring with his presence.
Wang Hao also impresses me a whole lot because I could never imagine that the world's best backhander is a penholder. (my conclusion comes from the effectiveness of his backhand in terms of how many points he scores from backhand per game; he also has an edge over Kreanga in the backhand-backhand battle. maybe we should discuss backhand in another forum)
As a penholder myself, I feel that penhold is very hard to play and extremely hard to master. So any penholder that becomes world top has huge talent.