The one where Waluigi and Wario get to vandalize Mario and Luigi's pictures.
But honestly, Mario Power Tennis remains the best Mario Tennis. It has a bigass opening cutscene that can fit well in a TV show, bloopers for said cutscene as a reward for being a tournament. The tournament has a trophy winning scene where it's always a pleasure to watch. Playing tournaments rewards your character too, as it unlocks a star variant that increases your character's power.
It's also the last game to have item battles. It's playing tennis but with a Mario Kart twist.
It also is one of the last games to have any sort of record-keeping. It gives you some incentive to play through the exhibition mode and murder those ace computers, and it gives some replay value that you'd completely lose if there aren't records.
The only thing Mario Power Tennis is lagging in is in the gameplay as the ball feels really sluggish and nailing nice serves just won't do anything. The power shots are pretty much hit or miss, but they end up really slowing down the gameplay sometimes. It's also really dated looking, as the shaders in this game did NOT age well. Why the hell are characters so shiny? Character balance is all over the place too. I don't really see value for Petey Piranha or Waluigi while you get Paratroopa whose lunge covers, like, an entire court, and lunges don't punish you as heavily as it does in the later games, and the game kind of sucks when you're playing 2 v 2 and the ai loves to lob spam you, where you also don't really get penalized for lobbing as you do in the later games. Oh, and Mario is aggressively mediocre like he usually is, though his offensive power shot is pretty fantastic as it practically negates the pushing from incoming offensive power shots.
Anyhow, Mario Power Tennis also lets you change controls too, so you can actually disable auto-lunge, but you also are prone to missing your offensive power shots (which, again, I didn't even believe was feasible until I watched a video of Mario actually missing the ball with his offensive power shot).
Mario Power Tennis also has actual match making settings, unlike Aces, and its stage selection isn't absolutely retarded as it is in Aces.
And finally, you can turn off the music in that game, but I think you can do it too in Aces.
The roster is pretty good though Mario Tennis Aces rivals it now. I miss Wiggler.
