When it comes to the 2020 Democratic Primary, I'm doing my best to follow Walter Lippmann's advice: I'm holding my opinions of the candidates lightly, and abandoning them (the opinions) gladly. My working assumption is there are a lot of good candidates in the race, and for the most part I'd happily support any of them, and my main priority is not getting so wedded to any one person that I grow antagonistic towards the rest. And so while I am starting to form nascent preferences, my "tiers" of support I think basically reflect this outlook.
Earlier, I gave my (extremely early-bird) predictions. This, by contrast, reflects my preferences (albeit preferences which account for who I deem to have a realistic shot at winning). More or less, I've divided the candidates right now into five groups:
Tier A: These are the candidates I'm most excited about or who seem most likely to garner my vote in the primary. Right now, this class is comprised of Kamala Harris, Elizabeth Warren, and Cory Booker.
Tier B: These are the candidates who have something in their profile detracting them from the top tier, but whom I'm still basically fine with. This would include Buttigieg (too inexperienced), Biden (too old-fashioned; too gaffe-prone), Sanders (too hostile; too fractious), and Klobuchar (too moderate). Still, if any of them win the nomination, I won't be cranky.
This tier also includes candidates who seem otherwise quite attractive, but for whatever reason don't appear to be getting any traction -- for example, Castro, Inslee, and Gillibrand. But if any of them do catch fire, they'd get serious consideration for "A" status.
Tier S: "S" stands for Senate -- this is the group of candidates who I'm annoyed with only because I think they should be running for Senate instead. This includes O'Rourke, Hickenlooper, and Bullock. In all of these cases, if they did somehow win the nomination I wouldn't be mad, but since I doubt they will and they're passing up races they actually could win that Democrats desperately need, I'm mad now.
Tier U: The "unknown" group. This includes the multitude of random people who are running for President for reasons best described as "inexplicable", and whom I have no interest in learning more about. Swalwell, Ryan, Yang, and Delaney fit in this group, among others.
Tier Z: The candidates I'm actively hostile towards. This is basically "Tulsi Gabbard", though to the extent that Mike Gravel is a living human and not a teen prank I guess he's probably here too.