Bismarck!
Though I admire Oni's spirit and caring plus...
Bismarck got better status (base stat) than Oni. In his match vs Duke/Fuji he was single handed facing then till 5-5.
Except for Fuji, neither him nor Duke didn't go all out.
Then he beats Tane, Konomi-San has his own way to tell us who is better than who. I mean yes, Bismarck beated Tane in his Tie break but the fact which makes us think the otherwise would be different but he kinda like do it this way as you know.
So yes, Bismarck is better than Tane, strongest defend player in Japan. ...
I don't compare Oni to Tane in order to get satisfaction:
Oni=Tane
Bismarck=Tane
Bismarck>Oni
... Oni got his powerup, so Tane (as for now and still Tane is number 2) but a part from that, let's take a look at Oni and Bismarck.
.... If you take a look Q.P vs Oni, Q.P was physically strong, but one of his great advantage was he uses his brain more than Oni.
He was outsmarted him, he has wide vision than him and the level that he shows about his instinct of tennis was way better than him, of course Kijin always isn't an answer, sometime you need to be powerful enough and widely aware of anything that goes on the court.
Q.P despite of everyone stopped chasing the ball, or sometimes his uses lob, or ...
I think the reason Oni's grip injured was the way that Q.P plays which make Oni to get in corners in various positions plus losing his balance then... This happens a lot in real world of tennis.
Bismarck in the other hand, he serves ace on Tane, over and over, then Tane tried to counter it, closing his eyes yet he couldn't.
Why? first I give credit to Tane, that he was a little bit under pressure which makes him rush to counter and boom.
But I saw Bismarck, his was superior to him, when they were in rally, Tane was barely answer his attack, while he was the one who serving on Bismarck, suddenly the pace changes then Tane uses his trick. Or the other strategy that Tane started, again Bismarck jump and counter Tane's Void then take almost the match.
Once again Tane changes his plans over and over, which was interesting to me, he always come up with ideas that I was thinking, man this time his can take this score, but there he was a better player.
Last time, he was playing vs Romeo and choose to go on tie break with him, and Romeo's stance was just like 120%... Still I don't know how strong Romeo is, but seeing this sence and Bismack/Q.P's conversation and again Konomi-San's (U R #2)
Bismack has strong base tennis, and I think smarter than Oni, who depend on his spirit and mucels.
Bismarck!