Dr. Will was a master manipulator during both of his seasons, and he pulled it off very, very well. During season 2, both of his close alliance members were voted out super early, thus causing him to manipulate all of the house to see him as a non-threat. BB7 aside, Will changed the game and made it what it is today, and without him this show would be nothing. He will forever be the face of this TV show, and he rightfully deserves it. That being said, though, America fell in love with will because of his character, who he was, how he portrayed himself, etc. Looking at BB7, had he not made that fatal final 4 veto error with Janelle and Erika, and he had made it to the end he would, with out a doubt, be the greatest of all time. But, he didn't, which is why I believe Dan is the best in my book. Not only did he get every single jury vote in his first season, but he also made it to the end twice. And, if I recall, the houseguests of season 14 said from very early on that they would not vote for a vet to win the game. I love me sone Ian Terry and all, but Dan should have won that season. Yes, his jury management could have been better, however it would not have changed anything since many had their minds made up from very early on. Also, Will played his second season among a cast of all-stars, and came in as a big target, but Dan was only one of four returning houseguests in his season (the only winner out of the four, might I add), making his target much larger.
That's just my opinion though, and I still absolutely love both of them and it is a very close 1 and 2 for me, but Dan just edges out Will.