This is a tough question because, as some have already said, there were a lot of very good races. All in all, it was just a really good track meet.
But for me there were two standouts -- the mens 5000 and the womens 1500.
Several have already commented on the mens 5000. Great race, lots of changes in the dynamics, awesome last 1600 (not just the last lap), but also still an amazing last lap too. And an ending no one expected. It was a classic that will be remembered.
The womens 1500 was also an incredible race. The Jenny - Shannon rivalry (or whatever you want to call it), the Brenda saga, lots of other story lines going in (Cain, Uceny, etc.), the contact along the way (but without the total disaster of the 800), and the incredible finish for third. Also, somewhat overlooked because of the finish for third, was the execution of a near perfect and extremely impressive race by Jenny.