The best degreaser/solvent

I just bought a set of turbines for my XJ off of a 89 Laredo. There are years of brake dust, dirt and grease that have hardened to a crust between the spokes. I used a standard grease remover like purple stuff and I got about 80-90% the stuff off. The remaining stuff is very stubborn. Any suggestions that might make the removal of the remaining crud a little more easier. Thanks

I generally use gasoline, honestly. It's far cheaper than most heavy duty solvents by the gallon, and you can use it to light bonfires after. You'll want to do it outside and wearing some pretty heavy duty gloves unless you really want skin cancer later in life. Diesel may work as well and is less likely to cause a fire.

Once the really heavy gunk is dissolved I clean up with denatured alcohol or acetone, depending on what I have on hand and what the paint or other surface finish materials involved will tolerate.

Since you already tried the purple stuff... Diesel, gasoline, or oven cleaner.