What saddle rail material is the strongest? I weigh about 120 kilos. Saddle rails bend.

I don't care about the weight of the saddle, just how strong the rails are and how comfy it is (I admire but have no desire to emulate people who cycle standing up. I have a bmi over 30, give me some credit for voluntarily cycling 20 miles a day)
As the diameter is standard (sort of) steel rails will be stronger than aluminium*.  But I'm surprised at saddle rails bending.  When I started cycling regularly I was about 100kg and even the roughest bits of road didn't come close to that kind of force.  You may benefit from a suspension saddle post for reasons of both comfort and damage avoidance.  Steel has a further advantage in that it's tougher than any of the other common materials for saddle rails, i.e. it can handle repeated stresses better without failing.

Do you ride very rough roads? If you get the occasional pothole or bump it's worth taking more of your weight on your feet just for a moment (you don't have to pedal while doing this assuming you don't have a fixed gear bike).



* Both carbon fibre and titanium saddle rails exist and are strong.  However they're usually used to make bikes light rather than strong and are unlikely to be available for anything other than road saddles (comfort was specified).m  Their brittle failure modes (carbon in particular) make them a bad choice in this case of unusally high stress.