Here, from that CNN/Selzer Iowa poll yesterday, are favorability ratings for the Dem candidates. I've also calculated a score on a Likert scale for each candidate (4 = very favorable, 1= very unfavorable), excluding voters who didn't know or had no opinion about the candidate. 
If you sort candidates into tiers based on that table, you wind up with something like this:
Tier 1a: Biden, Beto
Tier 1b: Bernie, Klobuchar, Harris
Tier 1c: Warren, Booker
Tier 2: Gillibrand, Brown, Hickenlooper, Castro
Tier 3: [everyone else]
Tier 1a: Biden, Beto
Tier 1b: Bernie, Klobuchar, Harris
Tier 1c: Warren, Booker
Tier 2: Gillibrand, Brown, Hickenlooper, Castro
Tier 3: [everyone else]
That list above ^^ isn't that far from how I'd rate the candidates' chances. I'd move some up or down a tier or a half-tier. Also feels like there might be room for another nonwhite candidate, especially on the left (Stacey Abrams?). But you could do a lot worse than those tiers.
