The nodes are considered 'best' in terms of its usefulness to 'you' ( in whatever ways) in the present.. Let it be in the past, present or future. So many discoveries of the past are virtually unknown to the 'whole world' at that time, but they are very useful today. Many of today's things are widespreaded (thanks to the digital media now), but useful to very few.

Best are considered from time to time and beg to differ for one person to another. Sites like Stumbleupon understands this very well.