Sheldon Cooper has the highest IQ among his friends on The Big Bang Theory. It's because of his intelligence that he can't help himself when it comes to assisting those around him. Sheldon has a tendency to educate his friends and colleagues without them necessarily needing his help. He automatically assumes that guidance is needed and that he's the only one who's capable of giving good advice.
On the contrary, Sheldon surrounds himself with very capable and bright people. They don't always need his advice and it makes him look pompous by assuming everyone around him is less smart. On the off-chance that Leonard, Howard, Raj, and co. go to Sheldon for advice, his words can either be incredibly helpful or problematic.
Best: Penny Really Should Have Checked Her "Check Engine" Light
For most of The Big Bang Theory, Penny wasn't in a great place financially. She drove a car that had a plethora of issues, but she never had the means to fix them. As soon as Sheldon got into Penny's car for the first time, he saw that her "check engine" light was on. He advised her to get it looked at but she put it off for years. Eventually, her car died and there was no point in reviving it. If she had listened to Sheldon early on, she could have had her car for a little while longer by taking proper care of it.
Worst: Shrodinger's Cat
Early on in the series, Penny finally agreed to go on a date with Leonard. However, she was unsure if it was the right move because it had the potential of ruining the amazing friendship they had built. She went to Sheldon for advice but quickly regretted it.
Instead of telling Penny what she shouldn't have or should have done, he suggested Shrodinger's Cat. As a person not interested in the sciences, Penny had no idea what Shrodinger's Cat was and how it applied to her. In this case, Sheldon's advice failed Penny. He was better off just leaving the room.
Best: The Relationship Agreement
Penny and Leonard have always had their ups and downs in their private relationship and in their friendship. And if they thought getting married was going to solve their problems, they were sorely mistaken.
After they were married for a while, Penny was upset that Leonard was letting himself go. The magic seemed to have disappeared. After a few days of arguing, Sheldon's dream came true: he wrote them a relationship agreement. He had been suggesting the option for years and the couple finally took him up on it — and it worked!
Worst: How To Ruin Someone's Christmas, According To Sheldon
There aren't too many people who give advice on ruining a person's Christmas, but Sheldon has always been an individualistic creature. In "The Clean Room Infiltration," he told Bernadette how to teach someone a lesson through the art of gift-giving.
He wanted to make Amy as miserable as she was making him, so he bought her extremely thoughtful gifts with hopes of her not having anything to give back in return. Bernadette never used Sheldon's lessons for getting back at someone, and it's safe to say that he stopped listening to his own advice after that night as well.
Best: Friends Can Help You Get Through Hard Times
It was devastating for the whole group when Mrs. Wolowitz passed away. She and Howard were practically attached to the hip; whenever someone went over Howard's house, they had no choice but to hang out with Mrs. Wolowitz — and eat her dinner too.
When she passed, Sheldon reminded Howard that he wasn't alone and that he too had lost a parent. While Sheldon didn't have friends to help him get through the hard times, Howard did. His advice seemed to help Howard during that dark time.
Worst: Caught In A Lie? Tell Another Lie
Sheldon isn't good with lying, which it was surprising that his advice to Leonard was to, in fact, lie. In "The Irish Pub Formulation," Raj's sister Priya was back in town and Leonard unexpectedly spent the night with her. He told Sheldon in confidence but was worried about the secret getting out.
Shockingly, Sheldon helped Leonard cover his tracks by giving him a backstory about spending the evening at one of Pasadena's "watering holes." The lie didn't work, Leonard panicked, and Raj soon found out that Leonard was sleeping with his sister.
Best: Don't Make Homemade Rocket Fuel
In "The Staircase Implementation," Leonard told Penny about how he and Sheldon came to be friends. As weird as he found Sheldon to be, he admitted that it was Sheldon's advice that saved his life.
Years ago when they first met, Leonard, Howard, and Raj were in the kitchen creating rocket fuel to pair with Howard's rocket. But as soon as Sheldon heard Leonard's formula, he knew it wasn't going to work. He advised them to get the rocket out of the apartment right away. Leonard ran the rocket to the hallway but Sheldon saved him by placing the continents in the elevator, kicking Leonard out, and saving their lives as the mixture exploded.
Worst: No Advice At All
Leonard found himself in girl trouble more often than not, and the only person he could turn to was Sheldon because they lived together. Sadly for Leonard, Sheldon didn't know the first thing about women or relationships.
In the sixth season, Penny started taking a history class and had to write a paper on slavery. She gave Leonard strict orders to stay away from her paper and to let her do it herself. Leonard quietly read the paper when Penny was sleeping and knew he had to edit it, but when he went to Sheldon for advice, he was at a dead end. Sheldon had nothing to offer Leonard besides "Psh, woman, huh?" and "Sucks to be you." Leonard left the table worse off than he came.
Best: Offer Someone A Beverage When They're Feeling Down
It doesn't matter if a person is sick or sad; when it comes to an emotional day, Sheldon always has a beverage waiting. Whether it's when Amy Farrah Fowler caught the flu, when Raj was drunk, or when Leonard was feeling blue, Sheldon's advice to anyone who wasn't feeling themselves was to drink a hot beverage that he was always kind enough to provide.
Worst: Lecturing His Employee About STDs
Even though Sheldon is a grown adult, he doesn't always know how to appropriately interact with those around him — especially women. In the sixth season, Sheldon had an assistant named Alex but his inept behavior drove her away. He was under the impression that she was inappropriately flirting with Leonard, so the only way Sheldon could nip the behavior in the bud was to talk about her "primal urges" and to offer her a pamphlet on STDs to deter her from acting on her alleged urges. In addition to upsetting Alex, this behavior earned Sheldon a talking-to from Janine the HR administrator.