Fascinating discussion. One can make a convincing case for either player.
Right off the bat, I'm going to have to say that a few arguments here have convinced me to admit that Ma Long has been more consistent by far and his earlier performances in big tournaments shouldn't be held against him as much as they are. His consistent level of dominance makes a very strong case and tips the scales in his favour.
One thing that I want to add is that ZJK's performance in 2011 Rotterdam is probably greater than any other performance by any player in the last 5 years. I've never seen anyone move so fast over the table. Though he struggles with this nowadays, it was no problem for ZJK to cover the entire table after playing the backhand flick. Even Zhang Jike said that his match against Timo Boll was the best he has ever played. His matches against Ri Chol Guk, Wang Liqin and Wang Hao were fantastic performances.
However, playing like that is extremely hard on the body and it greatly reduced his "shelf-life" so to speak, even if he gave us some breathtaking performances.
Ma Long doesn't use the banana flick over the table a lot of the time (which cuts down his footwork close to the table) and his style of borrowing power is cleverer and puts less strain on his body. This
post sums it up pretty well I don't think his domination is going to end until he is 30 and it is also dependent on Fan Zhendong's development.
Another thing that people forget to mention is how your
month of birth, your age and your compatriots play a factor depending on how long you can dominate.
Wang Hao had to deal with growing as a player while Ma Lin and Wang Liqin were at their best and then he had to fight against ZJK and Ma Long shortly after he starting playing his best table tennis (2009). He had to fight against 2 different generations, so to speak, and still made so many finals, which speaks volumes of his greatness as a player.
Ma Long, on the other hand, belongs to the same generation as Zhang Jike and Xu Xin. However, unlike WLQ and Ma Lin ( H2H 23-24), he has been dominant against his compatriots. ZJK hasn't been consistent and Xu Xin took a really long time to bring his backhand to where it should be. This is no fault of Ma Long's but it makes him seem more unbeatable than he really is as his teammates were not able to improve as quickly as they should have and it gave him a slightly easier time on the World Tour.
I can see myself getting roasted for saying this but at least I can hide behind my keyboard on the internet. I'm not saying that Ma Long's achievements should be undermined. I just want to say that he faced less stiff competition compared to the earlier WLQ-Ma Lin generation, who were both crushing foreign players as badly as Ma Long is right now but were true competitors.
His titles reflect this as well. Ma Long faced Fang Bo in the finals of the WTTC, faced a ZJK who was nowhere near his best in the Olympics and faced his junior Fan Zhendong in the World Cup. For me personally (and I might get roasted for this again), his world's title would have been sweeter if he had actually beaten Wang Hao and ZJK when they were close to their best in 2011. He was world number 1 the entire year in 2010 and was capable of doing so but he didn't.
His game was definitely not perfect prior to 2015. Wang Hao has shown multiple times that Ma Long can be beaten using clever tactics. Zhang Jike has also made Ma Long look beatable in the 2014 World Cup, 2015 German open and the China trials for the Olympics in 2016. When Xu Xin started to trust his backhand more, he was able to beat Ma Long twice in a row in 2016 and looked every bit his equal.
However, these are just things that detract from making Ma Long the clear winner in this debate but overall, consistency-wise, I'm still going to have to go with Ma Long.