Depending on the difficulty curve, casual, mid-core, or even just the Chaddiest among us. There's no one answer.
If the difficulty curve has reasonable steps that pull only small and easily surprises at a time, each well and intuitively indicated, with little space given to forget previously learned mechanics and little to memorize at any one point in time... casual players probably benefit the most.
Add on some intensive stuff that takes a step past mere accumulative learning and puts your awareness to the test... well, now midcore players will probably get the largest kick out of it.
Crank everything up to the memorization standards of Savage... actually, I'm not sure anyone especially 'benefits' at that point, as it'd just be "more of the same", but probably of lesser quality, for Savage runners.
Create new systems of progression (for classic example, Mythic+) whereby anyone can progress from any level, to their maximum given their gear and... everyone benefits. There's no exclusive minimum difficulty, and there's virtually no ceiling on the difficulty either.