If you look at the top of the Nielsen ratings, you will find CBS's nerd sit-com as the highest rated comedy. This is the greatest indicator that popularity does not necessarily mean best.
Once a clever and well-written foray into the minds of physics geeks everywhere, The Big Bang Theory has become almost a caricature of itself. No longer delving into the physics quandaries that made its initial seasons fascinating, the show has instead chosen to become a typical run-of-the-mill dating show. It is nearly impossible to distinguish it from one of the other friendship shows out there, except for the fact that the characters dress like they are in the 70s. Except that even that has changed.
Let's examine each character individually.
Sheldon - The genius that the show was based on, and also the reason for the early success, Sheldon has become quirky now rather than intelligent. His nuances are not because he is smarter than everyone else, or even a little socially awkward because he was in college at 15. He is now just goofy, with the writers even choosing to give him a girlfriend, something that Sheldon Cooper was always dead set against, not seeing the purpose.
Penny - Penny is and always was the meanest girl that has ever been passed off as a legitimate love interest to a nerd. Which, to be fair, is a pretty short list, but she would also probably make it on a list of girlfriends to non-nerds. Her character hasn't changed, but she has always been difficult to watch as she mistreats all the guys, especially Sheldon, who couldn't help his OCD. She's just mean.
Leonard - Somehow Leonard has become a ladies man that would give Leon Phelps a run for his money. He broke up with his Indian girlfriend in a subtle and non-decisive way, almost as if she had declared herself an atheist to the writers and they wrote her out. He dresses well, everyone loves him, and somehow he is the most popular guy in LA. Good for him.
Howard - Got someone to actually agree to marry him, even though he has the most over-bearing mother since Estelle Costanza. No girl who is in her right mind would find that attractive, or marryable. Yet here we are, on the brink of nuptials, and things are still going strong. Howard, meanwhile, is becoming an astronaut, which is fine, because that might actually happen.
Raj - Dresses nice, still can't talk to women, and is still consistently humorous. I have no problem with him.
The real issue is the way that everyone interacts. Sheldon was the only reason to watch the show at the beginning, since it was his personality that was different and funny. His reactions to everyday situations and scenarios was fantastic, and provided laughs each time. Once they ran out of social situations, they normalized Sheldon and the show became regular.
With Community, Parks and Rec, Modern Family, New Girl and 30 Rock ruling the networks, I would find it hard to rank Big Bang in the top 5 sitcoms.
But at least it's better than Whitney.