I think without a doubt, the best card player of all time is Stu Ungar, and if we're talking pre-2000's players, I'd also say he's the best poker player. Having said that, in the more modern era, you've got to probably break it down a bit more. For years Daniel "Jungleman" Cates was about as dominant as it could get in heads up play. Then with the popularity of nosebleed level tournament action in "high roller bowl" type settings, you've got the dominance of Justin Bonomo, and now Bryn Kenney. I find the latter two interesting because they sit atop the money list, but with a fraction of the tournament cashes of guys like Dnegs and Hellmuth.
I don't think I could come to an actual decision of who the best is or was. I can say, that personally the last people I want to see on my table in tournament or cash action, are Fedor Holz, Scott Seiver, Erik Seidel, or Phil Ivey anywhere near their prime.
Special shout out to my all time favorite player, Antonio Esfandiari.