I also think Sanders has the best chance to beat Trump, and at this point I’m rooting for him to go all the way. I think there’s a lot to be gained from having a clear-headed moderate person like Buttigieg or Klobuchar for President, but you’re right that Sanders would likely bring a bigger turnout.
When I said Trump was the least popular candidate, I was talking about his approval ratings during the campaign, which were always the lowest of any major candidate in the history of American polling. Hillary was right behind him. It’s true that he’s now the most popular Republican president in terms of approval ratings among Republicans. But that only shows us that there are fewer Republicans than there have ever been before.