I guess I have to show Tim's math for him...
Heads up against Seinfeld, I would say the main difference is their source of humor. Lucy got the broad laughs and while they were easier to get because TV was still mostly unexplored, that doesn't mean they came easily. The work the writers and Lucy herself put into ensuring they did what everyone else was doing only better and also things no one else had thought of attempting. By contrast, Seinfeld by necessity turned over rocks and found whatever comedy was hiding under them, which also takes the same kind of hard work and daring that Lucy had. Lucy ultimately gets the edge because of the broader appeal and relative simplicity of its comedy. Her bit with Harpo Marx is funny in its own right without having to know he and Groucho had done it before in their movie Duck Soup. Lucy didn't have to work on different levels and modern audiences don't need it to, either. It was well crafted, well acted and well executed. Because of Lucy, Seinfeld had to find its appeal in uncomfortable places, which it did exceedingly well, but it's still uncomfortable nonetheless.
As for Cheers, like I said in a different post, James L. Brooks' fingerprints on this, Taxi before it and Friends to a lesser extent after, are a distraction for me when it comes to this show. All of the characters had great moments (Coach was my favorite of all of them and had a couple of my favorites of the whole series) and the show had a loyal fanbase that was fully engaged in all the plots and subplots. Frasier Crane's evolution from one-note foil to getting his own spinoff show YEARS after being introduced is a testament to the depth and care the writers took. Despite all of its strong points, for me it was ultimately just another sitcom, albeit one of the better ones of its era, which was still an accomplishment to be sure.
Head to head with Seinfeld, I'd say Cheers has the edge in gravitas and pathos, but again I was still full up on those from Taxi, and many shows in that era were also going for those Taxi had done them so well. As strong as the secondary characters were on Cheers, they don't match the volume that Seinfeld had. Cheers had Cliff's mom, Seinfeld had Jerry's and George's parents, and even Kramer's mom etched her place in the show's legend by revealing his first name. Cheers was very good for what it was, Seinfeld was better. Cliff spectacular failure on Jeopardy wouldn't even crack George Costanza's top ten of humiliations. Seinfeld is simply better. Doesn't mean Cheers isn't good, just that it's not as good as Seinfeld.
It feels like I need to throw in an obligatory Seinfeld quote to put a bow on this package, but I try not to take obligations too seriously, so never mind.