Humor is subjective. And so is television, really. If viewership determined what shows were the "best," those of us who watched Community instead of The Big Bang Theory when the two shows were up against one another were clearly mistaken. Of course, many of us would likely argue that Community is the better show, regardless of whether or not it's more popular. So polls about the "greatest" series are interesting, but their value probably exists more in the discussion they prompt than in determining an actual winner. And I say this as a fan of Seinfeld, which is the "greatest sitcom", according to a poll 60 Minutes and Vanity Fair did. The Honeymooners came in second.
Below is a screenshot of the results of the CBS/Vanity Fair poll, which also includes the results of the current poll the site is running. Click the image to go to CBS News' page:
As you can see, Seinfeld seems to have lost its lead to Friends in the online poll that's running now. Friends fans have clearly rallied in support of the other 90's sitcom in contention.
TVLine is running their own poll and at present, Friends has 41.98% of the vote, with Seinfeld pulling in 24.84%
As is the case with any poll, it's worth considering who's being polled when looking at the results. From the two sets of results at 60 Minutes/Vanity Fair's poll, it looks like fans of the CBS show and Vanity Fair prefer Seinfeld, but assuming the second set of results is the post-poll poll, when opened to a wider audience of people, Friends takes the prize in popularity, "greatest" or not.
As for the results, I could hear arguments as to why Seinfeld, Friends and Arrested Development are all the "greatest" sitcoms. Seinfeld has the slight advantage of a bit more age than the other two. Friends is nearly as old though and still holds up just as well in reruns, dated or not. And Arrested Development, while never earning the kinds of ratings Friends and Seinfeld received, is also a pretty outstanding comedy series.