At one point, Conway started to rattle off some names of who he thought the top tier VCs were. Andreessen Horowitz, Kleiner, Sequoia, Benchmark, Greylock, Accel — Mike urged him to keep going as to why those are the best, but Conway said those are just the ones off the top of his head.
via techcrunch.com
I'd venture to guess what makes these guys who they are: class. In general, the best investors become that because they add value to the companies and treat the founders with respect.
It is that simple.
It is remarkable how many investors fall down on one or both accounts.