Warren, no contest. If I thought either Biden or Sanders were objectively good (not just eLeCtAbLe) candidates, I might've supported them in the primaries.
The most viable path to beat Biden for the nomination without a Biden own goal would have been an economic leftist candidate with a socially conservative streak. Think of the original Two-Income Trap Warren platform circa 2005. Such a candidate would have done exceptionally well in the COVID environment, too, likely having a bigger GE victory than Biden.
Going forward, and with younger candidates, the most viable outsider path is probably going to be a more polished/experienced version of Yang.