I apologize in advance, I will be using the term "core" in this post

As we know, in squats, dls, etc, power has to be transfered from our legs through our core. If our core is weak, alot of the power from the strong legs will be lost in this transfer.

what ab exercises do you think/has in your experience given the most carryover to the big lifts?

The reason I'm asking is that I'm kinda bad at training my core. And I'm pretty sure I should do it more. Squats and deads aren't always enough.

Oh and I realize "core" is really the lower back too.. but I'm mostly looking for ab exercises.

hhhmmmm...
I'm actually going the other way. I used to do a lot of direct ab work, but now I don't isolate them at all. and got to the point where situps with 70 lbs on my chest were easy, the only problem was that it's kind of awkward holding multiple plates on your chest. They want to slip around. Now I've gotten more serious about always including deads and squats and and I feel my abs being worked more than ever. I also just recently realized how much my abs feel worked after pullups..especially if I cheat a couple extra reps out each set

OK, this is more of an oblique exercise I think, but regardless, it's a nice twisting exercise. Twisting just happens to be one of the Six Basic Movements Al Schuler discusses in New Rules of Lifting by the way...hehe. Woodchops I think is what they're called. I use a cable, that is to the side and a bit above my head, and grab it with a handle. Basically, you are just twisting your trunk against however much resistance you put. It does a number on me...

For starters, anything heavy.

1. Full Contact Twists.

2. Stabilization Exercises - Various plank exercises, sometimes moving, sometimes holding.

3. Odd Object Training - Doing squats, leans, rotations with a ball, keg or sandbag. Anything where your weakest link is the ability to stabilize the weight and not strength.

4. Loading on Heavy Weight - I believe the Yoke walk is what finally taught me to use my core a lot more. Since the Yoke was traditionally over my squat weight. I know a lot of guys who practice walkouts with heavier than their max.

5. Odd Object Pressing - Anything that is awkward really. Press straight, to the left, to the right. (This is another stabilizing deal)

5. Using bands as added resistance - I really like this. They are easy to set up and when you need more resistance, just pull them tighter.

Hope this helped.

I usually do hanging leg raises with a dumbell held between my feet after deadlifts. This works your abs as well as your grip. And I do incline situps with a dumbell behind my head after squats.