If I only have time to do just 1 ab exercise, I would do bicycles. Typically crunches are used to focus the upper abs, then something like leg raises are used to supplement for lower abs, this exercise does both. Alternatives would be the double crunch.

Personally, I do weighted everything, abs are no different. I'd suggest this if you have time to perform a nice, short circuit. Something like weighted decline sit-ups(10-20 reps) to weighted oblique twists(10-20 reps) on the decline, to weighted leg raises(10-20 reps). Repeat 2-3 times.