The best concealer i've ever used has to be MAC's Studio Finish.
It covers extremely well. You only need the smallest amount on your brush to do every blemish and dark circle on your face. It's thick like a paste but it doesn't cake or feel heavy when on. It actually feels creamy and is lightweight, not heavy at all. Something I hate about concealers is when they go patchy or when your foundation lightens during the day and then you can see where you've put your concealer. This concealer just doesn't do that! It's got good coverage and it's long lasting.
The colour range is also huge so there is bound to be a colour for every shade. When matching skin colours for foundations and concealers, darker women struggle to find their shade, especially here in New Zealand where there aren't many Africans or African Americans. MAC is known for having a huge colour range.
Mine is very well used so as you can see I adore this product. I first bought mine in Dubai when I went a couple of years ago through their airport, which if you haven't been, is amazing. I would marry this concealer if I could, I'd have its babies even. It is that good. I've tried a few concealers, but none of them compare to the greatness that is this one. It's absolutely fabulous.