Saudi Arabia has the most oil on earth.
However, if you through oil shale into the mix, the U.S. would be at the top of the list by far. With over 2,500 gigabarrels of recoverable oil in that reserve (about 64% of Earth's oil shale reserves), you can see how we top the list. A few problems...it's not only very expensive to collect, but current drilling techniques prevent us from properly recovering it. Mining is the best method at the moment. We've known about it for a while, but only recently has any company gotten back into the game of trying to figure out a way of effectively drilling it, including reduce environmental impact.
Until we figure out that, Saudi Arabia will stay on top for now, with Canada, Iran, and Iraq following suit. It should be noted that while Saudi Arabia is on top by about 80 gigabarrels, Canada is expected to last longer by about 100 years based on current production.
Sigh...wikipedia for stats
a geology professor I'm friends with for the oil shale knowledge