1. Penny
Penny really brings something extra to the The Big Bang Theory because she is so unlike all of the other characters. While the rest of the gang consists of science enthusiasts who prefer to play it safe, Penny is a confident individual who's never afraid to take a risk or try something new. She is constantly encouraging her friends to try things outside of their comfort zones, and, as a result, she brings a lot of fun new ideas to their group. Those who consider Penny their favorite character are probably very similar. These people are the ones most likely to be asked to make plans for their friend group because they always have an unpredictable, yet entertaining, activity up their sleeves. Also like Penny, her fans have self-assured attitudes that enable them to always go after what they want and have a good time doing it.
2. Leonard
Leonard, an endearing and intelligent experimental physicist, is the people pleaser of the group. He always makes sure everyone is having a good time and feeling included when they are all hanging out. Additionally, unlike his other genius friends, Leonard has a pretty good grasp on what Sheldon would call "social conventions," making him easy going and fun to be around. Those who have picked Leonard as their favorite character are the most dependable of the friend group. They are the ones that everyone else knows they can call on in a time of need. Also, these are the people most likely to be asked to be a wing man/woman because they can hold their own in basically any social situation.
3. Sheldon
Sheldon's character is an ambitious and brilliant physics prodigy with OCD tendencies and an inability to pick up on social cues. He knows what he likes and he intends for everyone else to know it too. Though Sheldon is often considered annoying and brash by his friends and can be quite embarrassing at times, when it all comes down to it he is very loyal to his friends and always there to offer them a hot beverage when they are in need of one. Therefore, those who say that Sheldon is their favorite character are the ones who are considered endearingly awkward by the rest of their group. Also, though they might not do a very good job at showing it, they truly care about their buds.
4. Howard
Howard, a persistent engineer, is the funny one of the group, though his jokes are often extremely inappropriate. He is also very comfortable in his own skin and isn't afraid to own it. People who consider Howard their favorite character are the ones in a friend group who are never afraid to speak their minds. Everyone else comes to them when they are in need of an honest opinion or some tough love. They are also the ones most likely to make a "that's what she said" joke.
5. Raj
Raj comes across as the shyest of the group, but really he is often the one with the most to say. His character is super laid back and perfectly content with whatever everyone else wants to do. Though he can be naive at times, his innocence chips away at the cynicism of the rest of the group. Fans of Raj have similar characteristics. Their friends would probably describe them as bashful or chill. These individuals are also the most optimistic, and, thus, are the ones the rest of the group goes to if they need a good cheering up.