If you have tons of money and want to wait like 6 months (not exaggerating) -Lee
If you want decent filters right now for a low price - Cokin
I will say that the Lee ones are amazing. If you're selling photography and want stellar stuff then go Lee. I couldn't afford them, so I bought Cokin. They are amazing for the price, and their polarizer is just as good as anything out there, and I got mine for 35 bucks on eBay.
The Cokin grads and NDs will leave a huge purple color cast on long shots, but can remove them with software. I usually just end up making my shots bw to save time. I have talked to people that use Lee, and they say that you still get the purple, but I can't imagine that it is nearly as bad as the Cokins.
My advice is to start out with the cokin filters, and if it is what you really love you can always buy some lees that will fit in the same holder.
Just remember, Lee stuff almost always has a huge back order. You can find them on eBay, but if they aren't there you will wait. Also, lee is all glass, and and they break. If they break you will be sad. You will be very sad. Cokins are some sort of resin, and won't break. They will scratch, but they are like 20-30 bucks, so it's no big deal.