I don't why Mapbox's tiles would be pixelated (except that they might have the preview page misconfigured), but I don't think Mapquest Open supports retina tile

I don't why Mapbox's tiles would be pixelated (except that they might have the preview page misconfigured), but I don't think Mapquest Open supports retina tiles yet. A solution is to make the tiles half the size, but then all the labels are tiny. In addition to this, I dislike Mapquest Open's tiles because they're subject to jpeg compression. Unfortunately, it's the only free tile service out there without usage limits.