Character ranking:
Jerry
George (until maybe season... 5 or 6, when he was too much of a caricature)
Elaine
Kramer
They were all at their best in about season 3.
It's a rare show in that all of the principals were capable of carrying their storylines, and doing a little physical and absurdist humour.
My ranking would probably be:
1. Elaine
2. George
3. Kramer
4. Jerry
It's changed a lot in retrospect - when I watched it live back in my high school days, I liked Kramer the most, found George to be annoying, and enjoyed Jerry. Elaine was part of the gang.
I think my impressions have changed with time and watching episodes over and over again in syndication.
Jerry was the "straight man" for the most part, even he admits that. He's much more valued as a writer than as a cast member, but he did a fine job as the glue guy.
George was Larry David's proxy - he did a fantastic job, and I thought of putting him at #1 (it's super close), but at the same time, after watching Curb Your Enthusiasm, you realize how much he relies on Larry's perspective.
Kramer was the "wild oddball" but that's in keeping with how he's acted in all of his roles, going back to Friday's and UHF.
The surprising one is Elaine, who holds her own in stellar company and avoids the pitfall of the "rational woman character" that appears in most sitcoms. She's asked to do the most, in being the rational and intelligent one but also with her own bouts of absurdity and even physical humour that Jerry tends to stay away from.
Now it's not as common, but for awhile, a lot of female characters on sitcoms simply didn't have as much to work with and she stood out prominently.
Since then, you've had Tracee Ellis Ross on Blackish, Jane Kaczmarek on Malcolm in the Middle, and Wendi McLendon-Covey on the Goldbergs among others who arguably steal every scene that they're in.
I'm glad they've gotten away from the "comedian husband" with "much more attractive but eye-rolling wife" type of show exemplified by King of Queen's, Everybody Loves Raymond, the World According to Jim, etc. etc.
The best secondary character on the show (and arguably the best character period) was Frank Costanza - who manages to be absolutely hilarious in literally every scene that he's in. It was smart of them to use him sparingly, unlike his role on King of Queen's.
David Puddy also managed to deliver. Newman worked very well but again, only in small doses and appropriately.