I have played more Tetris than anyone can imagine. I have previously published posts about the ET Cannon and why it is incredibly powerful for perfect clears and sending damage.
But after much playing and experience I can tell you that even the ET Cannon does not lead to the highest possible score nor the most garbage sent to an opponent in vs mode.
The highest scoring method I have discovered is no doubt the Mechanical T Spin Double. More precisely, it is version 2. To get an idea of how it works, I have some videos here you can watch. Afterwards I will describe the math behind why it scores so high.
The reason that all my highest scores use this method is because of how all modern Tetris games score T spins better than a four line clear.
Tetris(four lines with I piece) = 800
TSD(clearing two lines by spinning T piece in place) = 1200.
Obviously 1200 is more than 800, but wait, since only two lines are cleared, this means that doing it twice would clear 4 lines and score 2400 right? Wrong! You see when you repeatedly do Tetris and/or T spin clears, you get a 50% score bonus. This means that the second T spin double would be 1800 points instead for a total of 1200+1800=3000 with only four lines cleared. This makes it the highest way to score in marathon mode where there are a limited number of lines to clear before the game ends.
However this method also is effective in vs battles. The reason is that the back to back bonus also applies to the garbage sent. T spin doubles send 4 lines of garbage but each one in a row after that will send 6.
There is one downside. Using this loop makes the stack slightly higher each time and there is no way to do only TSDs. This is why in many of my videos you see me place random pieces into the 3 wide hole in the middle. The key is that the pieces must be balanced. 1 of each of the 6 pieces(T,J,L,O,S,Z) must be placed. If they are not all used, there is an imbalance and it becomes impossible to continue the loop.
The I piece is a little bit different. Normally they are stacked on top of each other either on the left or right side of the screen. However because they become taller faster than any other piece, it is possible to have extras which can also go down in the hole. Sometimes this even allows for a Tetris clear. While this is not as good as a TSD, it does maintain the back to back bonus and so it is recommended to try this. A lot of the time it is best to wait until the stack gets very tall so that there is plenty of room to place the 6 pieces I mentioned in the 3 wide well. As long as this rule is followed, it is theoretically possible to play forever. Practically though, it only works until the game speeds up to about level 11 and the pieces drop too fast to place them where you need. At that point all other methods also break down so just freestyle and try to survive as long as you can.
I hope my videos and this post help people learn to use this most effective method in their Tetris games. The only method that could possibly score higher would be ST stacking. However it’s much too difficult for most people to pull off and it would result in getting KOd pretty easily in vs battles.
There is one thing that is even more high scoring theoretically than a mechanical T Spin Double loop. That would be an infinite T Spin Triple loop. However this would not apply in ultra or blitz mode because setting up a TST loop takes a lot more time and is not as easy to maintain. Still. If time is not a concern, then TST would be the highest score in marathon mode. I have not learned to do it myself but I am interested in the concept and it may be a future challenge. If I succeed there will be videos!