I always enjoyed the Normisms.
Unfortunately, Nick died long before Diane’s final seasons. 
If we didn’t have Rebecca we’d never have had Robin Colcord, so despite my preference of Diane as a character, I’m going to have to go with the Rebecca years.
I always found Diane annoying, and could never understand how anyone could find her likeable. Rebecca was a much more likeable character, and seemed to get along better with the characters in the bar. She “fit”, Diane did not.
And Rebecca was way more attractive.
Yes, Diane was annoying. I wonder if anyone actually found her likable as a person–I certainly didn’t. But that’s what I liked about her character. Well, not that exactly, but the tension between the somewhat condescending pseudo-intellectual college grad playing off the working class sensibilities of the bar folk created a fun dynamic for me.
Dianne, both the character and the actress who played her, were quite annoying. But it worked within the show. It added significant depth. Once she left, there was pretty much nothing left. It got really sad and pathetic quite quickly.
Kirstie Alley had done some terrible TV shows and movies and has acted badly far more often than well. But I admit she can actually do something once in a great while. E.g., Drop Dead Gorgeous and a small but interesting role in Nevada. On Cheers, though, she was doing a really crappy job.
To quote Olivia de Havilland: “Must I choose between a sapphire and an emerald?”
I pretty much agree with everyone above. 
Diane was the better foil, but she was so freaking annoying that I couldn’t stand to look at her sometimes!
Rebecca was way hotter. Overall, I enjoyed the Rebecca shows a lot more.
This is one of only a few shows where I felt like it never really jumped the shark. It was a great series from beginning to end. Furthermore it stands my ultimate test of good entertainment: I could sit down today and watch them again with no problem.