My favorite flat that you might still be able to find on ebay is the nike streak lt. The 3 and 4 were a huge downgrade from the streak xc 2 - lt 2 which were all about the same shoe. The lt 2 was the pinnacle of racing flats, but shortly after that it was decided that everyone bounces or something.
Anyways, other than that the best flats I have raced on the track in are the nike mayfly and mizuno wave universe. The new balance 5000 rc, reebok run fast pro, and some light flat adidas released seemed interesting but I was never able to get my hands on them - they competed with the streak lt anyways which was too amazing to go to anything else.
Nowadays the only flat you can buy retail is the type a9.