He was so intelligent and genuinely endearing. I loved how passionate he was about science, how excited he got over the smallest things, and how his intelligence never took away from his awkward, dorky personality. He was a genuinely good person who wanted to help people, and you could tell how much being the Flash meant to him.
I also feel like Barry lost some of his intelligence as the seasons went on, which is disappointing considering how much his scientific knowledge and skills were part of his character in the beginning. Early seasons Barry was a forensic scientist who could think critically, analyze evidence, solve complicated problems, and apply his scientific knowledge to situations other people wouldn't even know how to approach. He wasn't just fast because of his powers. He was smart, resourceful, and genuinely brilliant in his own right.
Honestly, it's hard to imagine the later seasons' version of Barry creating something like Gideon. Early seasons Barry had the curiosity, scientific knowledge, and technical ability that made an invention like that feel believable for his character. Later Barry sometimes feels like he's lost that side of himself, and I miss seeing him use his intelligence to solve problems instead of having so many solutions handed to him or relying on other characters to figure things out.
I miss when Barry felt like a forensic scientist who happened to become a superhero, rather than someone whose entire character revolved around being the Flash. His intelligence was such an important part of what made him Barry Allen, and I wish the show had done a better job of maintaining that throughout its run.