Buttigeig is polling around 0 percent with black voters. Andrew Yang and Buttigeig are stiff and dry and do not stand a chance in the ring with Trump. Yang does not have the rhetorical skill. He clams up at tough questions and will not challenge the status quo on healthcare. His words are empty. "Asian guy who likes math." - is not good enough to run on. Trump would wipe the floor with him. Buttigeig is too hawkish on the Iran crisis, and he's, like Yang, also robotic. I haven't seen anything from him that doesn't feel prepackaged (and his eyes give me the creeps incidentally - cold).
Bernie, even though he's a senior, is mentally sharp as a whip. He could do his job from a hospital bed if it ever came to that - and I expect he likely would. He would also pick a good VP aligned with his policies.
Biden is losing his marbles. I think he'd be a dangerous president (not as dangerous as Trump). He's stuck in the past and is no longer quick witted enough. But he has support due to familiarity.
Warren is good, she'd make a decent president, but she cannot win in the ring against Trump's difficult personality. That's the biggest obstacle in winning the election and only Bernie can do that - he has both the right kind of chutzpah, and good policies - in my opinion.