Okay, I guess anyone with a 99-yard TD run is "the best."
Only if you overweight YPC, which... You have a tendency to do, IMHO.
To answer your question directly... No. If a player is only on the field for one carry, and that one carry happens to be a 99-yard touchdown, and he never plays another snap... He's not in the conversation due to lack of sample size. He *might* be the best, but nobody is going to put their reputation behind claiming it because we can never truly know.
I think it is a sliding scale, yes. You have to account for situation, and offensive scheme, and the level of the players around him on a team, and the level of the competition he was facing on the other side of the ball, etc...
For example, look at Calvin Johnson. In his top year, he was 14th in receptions, 9th in receiving yards, T2 in receiving TDs. Overall pretty darn good, but not by any means a world-beating statistical season. And yet anyone who watched him on the field knew he was a man amongst boys. His production was limited by Reggie Ball and an option offense. But everyone could see with their eyes that he was special.
That same year Graham Harrell and Colt Brennan were the top of the heap statistically at the QB position. They were both good, but did anyone think they were all that special? Nope. They were system quarterbacks.
That same year rushing was led by Ray Rice-Rutgers (attempts), Garrett Wolfe-NIU (yards), Anthony Alridge-Houston (ypc), and Ian Johnson-Boise (TDs). Again, they were good, but did anyone think they were all that special? Ray Rice made a name for himself at the next level, but he never got a nickname like Megatron--he was ultimately most famous for punching his fiancée.
I wasn't paying attention to college football during Barry Sanders' year. I've never really studied tape or looked at the history during that time. So I have no opinion on him, either way. But he did win the Heisman, so at least SOMEONE thought he was pretty fucking good. Despite only playing one year.