Ohhhhh.
That's the trick. Which compound exercises are best for abs, if you have nothing but free weights and no gym access?
Any of the big popular compounds work great. When you body is either in open space, or having to balance heavy weight, your abs are constantly firing.
Standing Military Press -- Balancing weight overhead, abs stabilize
Chin Ups -- Body moving through space, abs fire to keep your core from swinging
Dips -- Same as chin ups
Squats -- Balancing weight and using abs to help push the weight up.
Etc etc for damn near any heavy compound or body-moving-through-space movement
If you are looking for the absolute best, I think that Front Squats and Zercher Squats may be king.
The problem with direct ab work is:
1) People rarely progressively overload. So they are just spinning their wheels in place with all them lame ass situps
2) Its painful and boring
fukk that. My abs protrude just fine and I have never done consistent direct ab work at any point in my life. They will grow with the rest of body if you do a lot of heavy compounds.