what are the durable and the toughest frame for xc/mtb?

This is a good question, but it is very hard to answer as posted. Every bike represents a whole series of tradeoffs between things like: weight and strength, comfort and efficiency, cost and practicality.

I have a friend who regularly rides a 20+ year old steel, hardtail mountain bike. As best I can tell, it is about as good condition now as it was when he got it. You can't ask for much better durability, but it is super heavy. Thus, for XC racing, he rides a much lighter aluminium alloy machine.

I had a hardtail Trek 6500 once upon a time. Again, it was a super durable bike, but my poor old body just couldn't take the hammering of a hardtail. I sold it a few years ago and got a full suspension machine. Interestingly enough, I was looking for enhanced comfort, but I have found I can ride much faster due to the more compliant handling of a full suspension machine.

My full suspension bike has proven to be quite durable, but being a racing oriented machine, the components needs a lot of tinkering to keep it tuned up.

Ffriends with titanium mountain bikes purchase them in the belief they will be the last frame they ever have to buy due to the durability of Ti. A titanium bike is also very light, but oh, the cost. You really need a big bank account if you want Ti.

So, I hope you see there is no simple answer to your question. In my experience, any well built alloy frame from a company known for building quality bikes will last you many, many years and take lots of abuse. I suggest you need to consider parameters like comfort, cost, weight, fit, etc. in addition to durability.

Hope this helps.