Wally, for reasons very well-stated by others. He really transformed as a character, not in a forced re-boot or re-imagining way, but in a natural progression from boyhood into heroic maturity. And his role in the Flash legacy is like no other, nobody else has quite inherited their mentor's mantle with the same success. Also, on a personal level, he's very relateable. He's not an angry bitter type but he has a hot streak to him, and he gets angry and defensive when his loved ones are involved, like I imagine most of us would be. He gets frustrated and enveloped in his own super-speed thought processes, he zones out, he struggles with his own identity as the next Flash in several ways, he grows tired of his teammates personal flaws, etc.
I like the rest of them too though.
I like Barry's simplicity. Someone said they prefer him as a monumental memory than an actual ongoing character and while that's harsh, there is some truth to it. Barry never had the benefit of modern story telling until maybe towards the end of his life (before being brought back, which hasn't been a success IMO). At the same time though, there's something refreshing about Barry's plainness. He's not (originally) someone who had their parents killed or suffered some existential tragedy; he was just a good guy who chose to do the right thing because it was the right thing, and used his methodical brain to help him do it. He didn't need a character trope like childish wit or darkness to remain important either. I think Alex Ross put it very well through the eyes of Bruce Wayne in Justice when he said "Barry Allen is the kind of man I would have hoped to be, had my parents not been killed". And his ultimate sacrifice, I mean, what else needs to be said? One of if not the most important deaths in DC history, back when it actually meant something to die, and for decades, the writers respected that, while still giving us tidbits of story, either through his past or in Wally's speed force adventures.
Jay is great just because he's the perfect grandfather figure for the Flash family. He's an awesome mentor and a natural co-leader in the JSA. His helmet is awesome, his vibrating face from the early years was awesome, he's just great. He and Barry both also deserve credit for pioneering the multiverse in "The Flash of Two Worlds" (which is more plot heavy of course but those two were the perfect characters to play the story through).
Bart is another one who went through a natural evolution. His stories with Max were great, he was a great member of the next generation of titans, and he was an important part of Wally's development as a mentor too. He grew up from Impulse to Kid Flash without losing his youthful edge, and by the time he wore the red suit and sacrificed himself, it didn't feel odd at all for his character to be as matured as he was, but also as lost as he was, never really finding his place in the aftermath.
Exactly my thoughts. I can't stand it, even though I watch the show out of some sense of obligation.