Usual brakes, front and rear!
because sometime i used single-speed too (i used a flip-flop hub) if the terrain too punishing
therefore both brakes is mandatory for me

but in case i have no spare brakes for my bike on the workshop (i seldom recycling unused bikes to usable bikes, so i have to make all parts used carefully) , i'll only use front brake and I'll find the rear brake another day, so at least it usable and safety