Whichever is already on the tractor. It doesn't look rocky or bumpy, you could do it with a riding lawnmower. If you let it get too long then I would prefer a hog to handle the extra length.
All depends on how often you want to mow. finish mow it and you will be mowing it every week. Bush-hogging it and you can mow when you want to, once a month keeps it pretty, twice a year keeps it looking like a field.
If you keep it as it looks now I thing a finish mower might work. My finish mower pops the shear bolt if I try to mow grass that is too high, so I use a shredder when it is. I could never understand how a bolt would shear instead of the belt just slipping, but it does.