Since the whole show is focused around the main characters (Sheldon and the rest), it is so easy to forget about the side characters in the story, but there are some that have almost the same performance as the MC.
Like: Mary Cooper, Stuart Bloom, Barry Kripke..etc. Here are the seven most entertaining non-main characters that it’s hard to imagine the show being successful without them. they are like the final touch on a piece of art.
Mary Cooper
Sheldon’s mother was one of the funniest characters in the show and the most offensive one. But despite that, her jokes were very entertaining.

Like that time when She asked Raj about the chickens and if they are what the Indians think it’s magic. Or when Sheldon named all the cats in his apartment then she said: “ok you got a lot of cats and you give them cute Jewish names.”
President Siebert
I think this character was the most patient one with Sheldon at work. Most of the time president Siebert was keeping Sheldon and Barry from destroying each other, despite all the problems they caused. Maybe Siebert wasn’t as funny as the others but he did great as Sheldon’s Boss.

George Cooper
Sheldon’s Big brother was a very important part of Sheldon’s past. He played the role of the father after their dad died. his obsession with tires is quite unique, as also shown on the Young Sheldon Show.

The day of Sheldon’s wedding is a very important event in the show, so having the entire cooper family is good. Because it also shows what the relationship between the two looks like and how Sheldon will behave in front of his big brother.
Alex Jen
She was supposed to be a brilliant assistant that will assist Sheldon through his experiments and studies. however, he is an unpredictable person. This side character helps exploit the boss side of Sheldon at work.
If you have ever been wondering how is it like to be working with Sheldon, the episodes of Alex Jen shows exactly that. Instead of having an assistant do her job, he treated her like his servant to do his personal duties for him like buying a gift for Amy or a new game kit.
Stuart Bloom
Even though Stuart was kind of the most pathetic character in all of the show, it really shows how his friends and loyal customer Leonard and the rest help him change himself. Also, it was so funny watching his remarks and how he thinks about his life.

It was a unique character that is both funny and drives emotions, like sympathy. One of the best Big Bang Theory Stuart Moments is: when Raj tries to take pictures of him to upload them to Stuart’s profile, the image before shows what kind of poses Stuart did.
Barry Kripke
Sheldon’s rival at work. The most entertaining moment hands down is when Barry put Hilum gas in Sheldon’s office while he was being interviewed on a radio show.

After that Sheldon’s voice changed and got funny, making everyone in the show laugh as hard as they can. that was the time Kripke humiliated Sheldon on a national radio.
Barry was ranked the number one side character on most of the sites. it is always good to have competition, especially against someone like Sheldon cooper.
Mrs. Wolowitz
Even this Side character died early in the show because the actual actor died, it was a blast to hear her funny voice and ignorant comments, even though it was so loud. And just like Sheldon says to Leonard: “I will miss hearing her voice again.”

After that, the show, for me was missing something even though it continues to grow its fan base. I don’t think that without her Howard’s life will anything near funny as it was.
Final Thoughts: There were more than these that are worth mentioning but the characters in this list are the most important ones after the main characters. If you like to know more about The Big Bang Theory Show visit this section on the website.