If you want a dedicated core workout try some sand bag get-ups (also called turkish get ups I think). I usually do 3 or 4 sets once a week. They will work you! Planks are good as well.
Planks (front, sides, back; weighted or not). Hollow body holds too.
Regarding above comments, concur that if you do the core compound movements, you shouldn't need to devote a lot of time to core work. Stop doing situps or crunches constantly, unless you'll be tested on them (military or LEO community). Other good weighted movements are goodmornings, Romanian deadlifts, and farmer's carries (esp. asymmetrically loaded).
Turkish Get Ups with a kettle bell. There is no better home fitness equipment than a kettle bell. Versatile, effective, and takes up very little space.
Most of my core is carrying a pack but I also rotate a few routines I learned from various mountaineering sources. My favorite one is from the book training for the new alpinism. It's mostly stability stuff that looks easy but is really pretty hard.
There's literally millions of youtube videos if you search core or ab workouts.
I never really follow along with them but it's a good way to find some variety in your personal routine.
In the military, so I do a lot of standard core exercises as I do get tested on it as SWOHTR mentioned. I lift 5x a week, run 3x a week, do core circuits 2-3x a week. I like core circuits though, and I'll do a circuit between my lift supersets.
Laying down fully extended, lift legs 6" and hold for 10 count, do 30 leg lifts then return to 6" hold for 10 count, 25 lifts and return to 6" for 10 count, continue dropping by 5 until you reach 5 lifts, then hold for 10 count. Dont let your legs touch the ground.
25 crunches.
Then planks centered, left, right, holding each for 30 seconds.
25 crunches.
Then hanging leg raises to failure. Try not to swing your body, but lift your knees to touch your elbows.
Do this circuit 3x throughout your lift routine. You'll feel it...
The only dedicated core exercises I do are planks and flutter kicks. I like the flutter kicks as they hit hip flexors better for me and it's easy to have issues with those if you ride a desk for a living...