I think most riders who fall into a chair seat and struggle to get rid of it are in a saddle that doesn't fit somehow. It is either off balance due to poor fit on the horse or the stirrup bar placement is wrong for the rider. It may not be your fault at all, but the saddle's fault, and knee rolls and blocks make no difference in your equitation except to force your leg into the right place at the expense of possibly causing you pain or covering up a saddle fit issue. Knee rolls and blocks can be great for added comfort and stability, but you should be able to find that perfect equitation in a well-fitted flat saddle too! And I believe flat saddles are better for teaching a rider to have an independent seat and stabilize and strengthen their own leg. I think flat saddles make better riders, and expose poor saddle fit! I wish knee rolls and blocks weren't so much of an "in" thing right now!
Unfortunately, because saddle fit is so individual to BOTH horse and rider, there is no one brand that is best for every horse and rider pair, as every horse and rider are different and have different needs.
I suggest you do a little research on saddle fit, and do a self-evaluation on your own saddle. I think you'll find that your current saddle just doesn't fit.
A good way to figure out if your chair seat is a problem on your end or because of your saddle, try riding with our feet out of the stirrups for a few laps at the walk and trot. Settle into a position that's comfortable for you, and then look down at your stirrup without moving your leg. Is your stirrup hanging by your foot or is it hanging in front of your foot? If it hangs in front of your foot, the saddle is the problem, not you.
Here is a good resource for evaluating saddle fit: Saddle Fitting - 9 Step Guideline for Optimal Fit
If you find that your saddle doesn't fit (which I think you will), then it's time to go shopping! If you have a high budget you should go the custom route and try out some saddles like Schleese, Delgrange, and County. Decide which brand feels best for YOU and then call your nearest representative to have a saddle custom fit for you and your horse.
If you're on a limited budget (like most of us) do as much research on saddle fit as you can. Take a wither tracing: Wither Tracings - How to Make a Wither Tracing for Saddle Fitting
and then go to as many local tack shops as you can to try saddles in the store. Only consider saddles that fit your wither tracing and your budget (though it is fun and educational to sit in other saddles). Sit in them in the store and see what you think. You should have about a hand's width of seat in front and behind you. The seat should not be so narrow that you're sitting on seams but it should not feel too wide either. Your knee should be able to rest in the correct place on the flap etc. Keep in mind that the dummy horse in the store is not the same size and shape as your horse so the balance of the saddle will not be the same on the dummy as it will be on your horse, so take any saddles you think might work home on trial and evaluate the fit based on the 9 point videos and ride in them. If the saddle is well balanced and your leg wants to fall naturally into a good position, with and without the stirrup, the saddle just might work. Then you need to hire a saddle fitter to evaluate the fit of the saddle before you make the purchase. The saddle fitter can also help you decide between more than one saddle if you are having trouble making a decision.
I recently went through this process with both of my horses. I ended up with a used Collegiate Dignitary for my Shire cross mare. It wasn't my favorite in the store, because it was too wide for the dummy and was tipped too far forward, but it fit my wither tracing and my budget so I took it home to try. It was perfectly balanced on my own horse, super comfortable and put me in just the right position. A saddle fitter gave the stamp of approval and now me and my horse are both much more comfortable. I had trouble with a chair seat in my old saddle because it was too narrow for the horse and was sitting off balance. Now I have no trouble riding with my leg in the correct position!
My husband ended up with a used Stubben Siegfried for he and his appaloosa gelding. Their riding progressed in leaps and bounds after finding a saddle that didn't pinch his horse's withers!
So, in my experience, the key to good equitation starts with the right FIT of saddle, not any specific brand!