While technically not a Democrat (an Independent), Bernie was a great candidate in 2016, it's just that he got sabotaged every which way by the establishment Democrats in favor of Hillary because it was "her time", ignoring the political realities. He'd still be a good candidate for 2020, but there may be some other good ones too. Though not every Dem who has made noises about running will be a good candidate of course ... including those I otherwise like such as Beto O'Rourke - he's too nice to go up against Trump (and possibly too nice to up against any post-Trump GOP candidate who might emulate him). On the other hand Richard Ojeda vs Trump would be an interesting fight to see unfold.
Also, to say it was "the Democrats" who ran this new strategy of running every partisan seat would be misleading. The establishment Democrats didn't do this or want to do this, it was mostly new people. The establishment has been happily getting paid to lose for years. In fact, a lot of establishment Dems were challenged in the primaries by more progressive candidates too (not just red seats), and naturally they closed ranks in support of the old guard. Sadly many of these challengers didn't make it (though sometimes because they actually weren't effective candidates, more often due to the party putting their finger on the scales by working against them actively).