I absolutely think it should be the last cookie the player downloaded that should be paid. Two reasons.
1. Assuming it's a level playing field and no-one cookie stuffs - as in all other areas of sales and promotion closing the deal is key. The last site was the one that gave the player a reason to finally sign up - they should get the sale.
2. Knowing that cookie stuffing does go on then there's even more reason for the last site the player downloaded a cookie from to get the sale. They prob visited the evil cookie stuffers site and left without clicking anything but they are now carrying a collection of different site's cookies. They then visit a quality site and like the review, bonus, rakeback, whatever, of a poker room, they then click the link and sign up. It's obvious who made the sale and who should get the credit - the last place they visited.
If cookies were automatically overwritten with each new click then not only would it be fairer, but it would be one in the eye for the cookie stuffers and they would lose alot of their business overnight.
It IS fair and right that if a site has a deal that the player wants, then they should be able to give that player advice as to how to ensure they get the deal they want. I know this from experience - when I first started out as a poker player I wanted a 'free gift' of some software from a well known site - I happily signed up at a room I had never heard of at that time and started playing, waiting for my gift. I later found out I had been cookie stuffed and I was under another affiliate and there was nothing that could now be done (up until that point I had no idea about cookies / affiliates). I carried on playing there 'cos I loved it, but it was that free software that made me sign up - they should have got the sale and I should have got my software.
Wish I knew who it was!