I've seen most episodes of every season more or less through reruns, tapes, steaming, and just watching it every week. Some seasons are more tough to find than others, but I've seen most all of the episodes that matter.
I think in terms of star power, the first 5 seasons are hard to beat. Radner, Murray, Chase, Aykroyd, Belushi, Curtin, and Morris were all extremely talented and most of them went on to superstar status. There were some great individual sketches from those years, and when you had someone like Steve Martin or Buck Henry hosting, there were some fantastic episodes. And it was groundbreaking stuff at the time. That said, several episodes of that era are basically unwatchable today. That's especially true of the first season. It mainly had to do with the show getting its legs and figuring out what it wanted to be. Still great stuff and probably its strongest era.
I do think that someone who dismisses everything on SNL post 1980 is missing out, though. The 80s were a pretty weak period for SNL, but Eddie Murphy had some funny stuff. I'm not a big Murphy fan, but my goodness was he talented.
Maybe my favorite period of the show was from about 1989-1993, which is when you had Phil Hartman, Dennis Miller, Dana Carvey, Jon Lovitz, and Jan Hooks anchoring things with Chris Rock, Mike Myers, Chris Farley, Rob Schneider, David Spade, and Adam Sandler coming in soon after. For my money, Phil Hartman was one of the top 2 or 3 sketch talents the show ever had.
I always liked Will Ferrell, and there was lots of good support during his tenure too. Since Ferrell left, Bill Hader was probably my favorite cast member. Tina Fey and Amy Pohler are national treasures as well.
Right now they have a very young cast, so it might be another year or so before it's worth watching. Kate McKinnon is probably my favorite right now followed by the little-used Kyle Mooney. Anyway, it's safe to say I'm a SNL lifer, so I'll be watching. It's a weird show; it's probably been called "Saturday Night Dead" for more of its history than not but when it's on its game, it's really good stuff.