If you want the best answer to looping chop it would either be boosted H3 or Tenergy 05. H3 is hard and T05 is medium hard and neither have "sparse" pimple structure.
But either of those rubbers you have to be able to handle.
In comparison, T64 or T64fx have a sparse pimple structure and T64fx has a soft sponge with that pimple structure and neither are close to as good for looping backspin as T05 or T05fx because they both have a low throw angle which means you have to work harder to arc the ball more. It also means they have a high speed/spin ratio. Whereas, to loop backspin, you want a high throw angle so you can make a high arc as a result of the high spin/speed ratio.
You also should know, you aren't going to achieve success in topspinning backspin if you don't want to learn to loop backspin with a high level of consistency.
What I would actually recommend to you is a rubber like Donic Baracuda which is slow and very spinny and good for refining your loop vs backspin. And then do a simple serve and receive drill a lot.
Here is the recommended drill:
1) you serve short backspin
2) training partner pushes long to the FH side
3) you try to loop
When you are solid with that you can make the push placement anywhere from the middle of the BH side to the wide FH and you have to loop with FH. A good player should actually be able to loop with FH on a long push anywhere from BH corner to wide FH solidly. But that comes after being able to loop backspin solidly when you know where the ball is going.
That kind of drill can really help you get solid at looping backspin with more confidence. At a certain point you will use it as strategy where, if you can get your opponent to push long, you will feel you can take control of the point.
By the way, it is worth doing both sides of that drill so your training partner gets to open the loop vs backspin also.
Good luck.
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