There are much better systems than D&D in the world. In fact, D&D is rather incoherent and riddled with irregularities, despite its charm. Yet from a certain perspective, D&D is the best game there is, simply because it is the first and most popular.
This is no circular argument: what I mean to say is, because D&D was the first role playing game and has hence secured the Top Dog space for itself and ruled the roost ever since (to mix my metaphors just slightly), there are vastly more people playing it and hence vastly more people creating things for it. To put it in fashionable terminology, the D&D brain trust is massive in comparison with that for any other RPG. The OSR blogosphere is only one small example of this. From house rules to new monsters to campaign settings to alternative classes to whatever else you can think of, there are dozens of times more people creating things for D&D than any other system, and thus there is a greater vat of accumulated knowledge from which any given D&D player can draw inspiration and edification than there is for game X, Y, or Z. Or even A, B or C.
But it doesn't stop there. I would suggest that D&D's pre-eminent position at the forefront of the hobby has a more profound and subtle impact on D&D players than just providing them with loads and loads of ideas, new rules, and customisations. The boon is far greater, if slightly more difficult to access: it comes in the idea that D&D is not a fixed thing but is ever changing. The fact that every month there is (was, once) a new Dragon magazine packed with different rules, that there are blogs and forums and websites out there dedicated solely to the creation of new material, that a never-ending sequence of splatbooks was being farted out by TSR at its height...all of this creates a self-perpetuating, virtuous cycle, in which any individual DM or player is being constantly reminded, almost as if he can't escape the notion, that D&D is ever evolving, ever-changing, ever-ready to be kitbashed and mangled and beaten into something new and different and better. This creates a special D&D mindset. The pressure to think about evolution. Other games may have a certain feeling that the rules and content can change, but D&D is impossible to imagine without the constant pressure of change, improvement, expansion. And this D&D mindset is a precious gift to the player of that particular game. It means that anybody playing D&D is always confronted with the question: what can be done to make this better?