Critically Fox rivals Marvel, but commercially its still slightly behind DC.
I imagine if Fox have a clear vision for where the take the X-Men franchise and can execute it well they'll eventually be the dark horse and actually be able to rival Marvel's blockbusters commercially. Hell, they might end up in second regardless if DC messes up badly enough.
If we're talking quality, I guess I see them as the two main rivals. I prefer the X-Men films to the MCU, but Fox also helms the Fantastic Four films so that kinda balances them out.