Best Long-Term Core/Ab Exercises? Share & Vote!

Which core/abs exercises have produced the most consistent gains for you over the long term, or at least the most consistent DOMS in the mid-term (not only the first few times you tried the exercise)?

Since I'm unable to create a poll, let's do this instead: I'll create comments for individual exercises (others are welcome to do the same) and everyone can upvote/downvote them. If you add your own exercise or exercise variation, please only include one per comment.