My abs are shit too. Just today I tried a simple ab superset with three exercises, and my abs went into spasm right after because they are so under-utilized... couldn't even get another set off. Try these movements out for core work... I understand how planks (front/side) are good, but I recently read a livespill comment by Thibaudeau on ab work and what he does, and my abs are already way stronger.
1) Do straight sets of more demanding or technically complex movements: L-holds, dragonflags, back extension ab holds, hollow rock.
2) Do supersets of the simpler movements (pick 3-4): cable crunch, swiss ball crunch, hanging leg raise, knee-to-elbows, lying garhammer raise, etc).
3) Ring work! If you have access to rings, you can start doing progressions (search on youtube) for back/front lever, maltese hold, skin the cat, and iron cross. I started doing full reps of skin the cat, and I've never felt my abs/lats work so much in unison. The way I felt, I was like, "this shit's got to be healthy as f*ck!".