XC mountain as in my 29er XC MTB. It goes where gravel bikes cannot. Where I live, we have truly harsh rocky technical single track etc that is very demanding.
We also have a lot of gravel trails, the 29er is adept at that too, simply lock suspension to reduce energy losses.
The question is however a tough one. I chose my MTB simply because it can do everything and I'd not want to be restricted by a specialist bike if I was only permitted one.
My road bikes are my favourites, I prefer road overall. But not for touring, commuting - I am a speed merchant so my road bikes are suited to that pursuit.
I have never shared the joy many have with steel bikes, they are very much 'meh' for me. I own one! I raced on steel in the 80's/90's. Not for me now.
Gravel? Yep, very good, like CX, but the rocky terrain I take my MTB over is just too harsh without suspension. So my pick would have to be my carbon 9kg 29er full sus XC MTB with XTR in this instance. (Not downhill longer travel, heavier and unnecessary for me)