Yeah I find ass-to-grass squats very good for hitting the abs... i.e. go as low down as possible with the bar. Deadlifts are excellent for stregthening the whole body.
To hit the lower abs you can do hanging leg raises but I hate those. I do a mixture of two exercises as I find it hard to really target the lower abs without using other muscles like the hip flexors.
I'll lie on my back on a bench, then bring my legs up so as to do a leg raise, then I'll curl my hips and legs up using my lower abs, then I'll slowly lower and repeat. Absolutley murders my abs!!