Favorite Core Exercises?

What are you favorite Exercises for Core? Trying to get my 6 pack on this summer!

I like leg raises. My lower abs are always a little behind and then turn my legs to hit the obliques.
Good topic, I'm interested to see what people say. I would like to try some new things. I've used an app on my phone before too. Step by step videos with like 5 different exercises for about 10 min.

I do Leg Raises with 10-20 lb depending on reps, I do 8 min abs and a 20min uphill treadmill walk before every lifting session to warm up. I also do squats and cleans.