I figured some of the SNL fans around here may enjoy this.
http://grantland.com/snl/
It's crazy how many talented people have been on that show over the years.
The problem with rating SNL cast members is that people always retroactively glorify them once they've left the show for post-SNL superstardom. I watch the show religiously... take someone like Will Ferrell for instance. When he was actually on the show, there was no shortage of people complaining about how annoying some of his skits were. "All he does is scream!!", "he's not funny!", "that cheerleader sketch is terrible!"... but then he goes on to a movie career and everyone claims he was brilliant on SNL. The previous generation is always funnier than the current generation until the current generation becomes the previous generation. It's rare that anyone has enough perspective to actually accurately measure a member of the cast in the moment... partially because there aren't a lot of people like me who watch the show consistently every week.
Truth is, there's always plenty of crappy sketches on the show... but there's also plenty of hilarious ones too. You remember the hilarious ones from previous generations, because those are the ones that usually stick around.
For what it's worth, I think Bill Hader was my favorite cast member of all time. He was consistently great, in my opinion. He's yet to have a huge break-out post-SNL career so I doubt he'll survive the later rounds of that Grantland poll when matched up with some of the "legends". Compare him to someone from his own generation... Kristen Wiig. She's brilliant in pretty much everything she does outside the show, but on SNL itself I found her consistently grating and annoying. Too many of her characters veered into "MAD-TV" territory. And yet, she'll probably go further than Hader, because "Bridesmaids" was a huge sensation and she's arguably a much bigger star already.
I just feel like every time a discussion about the validity of someone's SNL career comes into question, people typically weigh the sum of their entire career. I'm sure Fred Armison (who was actually pretty funny on SNL) will have a bunch of extra votes in that Grantland poll, because "Portlandia" is a huge hit right now. And folks will look at cast members like Taran Killam, Kate McKinnon and Vanessa Bayer (who are actually pretty [dang] funny) as terrible, because they are the current generation.
I'm telling you... it would be interesting to do this exact poll again in 5 years. Bill Hader recently signed a developmental deal with HBO to create and star in a new TV series. If that show ends up being a hit (I hope it does, I love that guy), I bet people retroactively claim he's their favorite SNL cast member of all time.
That said, it's going to be hard for anyone to beat Murphy, Belushi or Hartman in that poll.