The best, for me, is Kat Von D's Lock-It Tattoo Foundation. I have a fair amount of scarring and discoloration from years of battling acne. The battle has been won, but the scars remain. This foundation is the only one I've ever found that covers all of my scarring and disocloration without looking and feeling like a mask. I don't even need to use concealer with it! The only downside is that the lightest shade is too light, and the next shade up is a bit too dark, but I fix this by mixing in a little Revlon ColorStay in the lightest shade (I forget its name) to lighten it up just enough. It keeps my skin matte and is transfer-proof throughout the day. I use my Real Techniques by Samantha Chapman expert face brush to apply it, and it looks absolutely flawless!
As for the worst, I'd have to give that dubious distinction to that Cover Girl and Olay stuff -- the one that Ellen DeGeneres used to promote. I think it was called Simply Ageless. The packaging was difficult to open, the product ran out quickly, the colors looked unnatural, it felt heavy and greasy on my skin, and rubbed off onto everything I touched. It felt like wearing a mask made of grease and oil.