In order to argue who is the best, you have to stablish parameters, as simple as that is. Who is the best at winning tournaments, probably walder that i believe won the most so far...... BUT...
Like in football, a lot can be said about that. The 1st football world cups had a handfull of teams participating, it wouldnt be the same winning a world cup back then, than in the 80's , or in the 80s as it is now. Sports everywhere have evolved so so so much in every aspect, and there so much competition out there, that the ppl that excels now in any sport, has to be a perfect machine in that sport in wich the variables are now more in number than ever in history
Bearing all of this in mind. Ma long and the other top 5 chinese players of THIS moment, would kick waldner ass at his peak 6 ways to sunday. I'm not saying waldner wasnt epic, he was for the 80s and part of the 90s, but the sport, the phisicallity, the technique, the extreme training process and quallity of trainers and actual top of the line partners to train with. Its the best now than it has never been, by a loooooooooong shot.
Waldner wouldn't even know what to do with a fan zhendong flicking every single ball he serves, wouldn't know what do to with a ma long that can attack ANY ball and has such an impressive eye for being able to read their opponents and counter spinning their attacks not only with amazing never before seen power but flawless technique.
Those who think ma long is pure brute force, have no idea the amount of technique you have to possess in order to be able to attack everything to perfection like he does. I have seen ALL of Waldner matches, he didn't even have a proper backhand topsping attack weapon like zhang jike or fan zhendong have nowadays, he wouldn't be able to stand a rally against a xu xin that can even backhand topspin around the net balls, because he never had people around him that took the sport that far away. He was a total professional with great technique and he had a smart way of approaching the game, he was the best of his era, and this era is way harder, with a lot of more insane skilled people that simple took the game to be the insane show of technique and power that is today.