As a spin-off of the 100 push up thread, I'm wanting to add an ab version that's similar. With that in mind, what's the best one or two ab exercises that are doable without weights/plates/gym? Sit-ups? Leg-lifts? Crunches? Butterfly-kicks? Planks?

If you're looking specifically for the abdominal muscles, I think situps with an abmat is great. I've known people with great core strenght, can do tons of situps, plank for a long time, etc, But when I introduced them to abmat situps, they have very sore abs the next day.

If you're looking for core strength in general, probably some sort of dynamic/high tension type of plank would be good, along with abmat situps, and maybe leg levers/lifts (lot of hip, but still an important core movement in my opinion).

While the workout below has some of the better exercises I have done and continue to do weighted workouts actually make my abs more sore. Things like front squats, single handed suitcase carry, throwing a sandbag, etc. actually activate your abs more than most traditional ab works.

https://www.uphillathlete.com/scotts-killer-core-routine-pdf/

This is a wonderful all-around core routine.

Freestyle wrestling ;-)

Knees to Elbow / Toes to Bar
Ab Wheels

Racquetball. Hill workout. Pullups. Inclined crunches. Eat less.