So I think we're looking at Udall, Carey, and maybe Scoop Jackson as your top-tier candidates; Hart, Glenn, and possibly Jerry Brown as the "young guns"; Askew and Mondale as the "resume" candidates (a la Bill Richardson and Joe Biden '08) waiting for someone in the top tier to slip up; and then a whole bunch of fringe candidates like Alan Cranston, Fritz Hollings, Sam Nunn, Dolph Briscoe.
Let's go through these candidates:
Udall -- found out he had Parkison's in 1979; likely won't run
Jackson -- fact is, his hawkishness during Vietnam prevents him, with post-McGovern Democrats, from ever winning the nomination
Mondale -- too associated with the "blown" 1976 election; unlikely
Askew -- not likely
Carey -- perhaps
So that leaves the "young guns", Hart, Glenn, and Brown (and the fringe candidates) -- who now look much more like top-tier material:
Hart -- he's got a lot of good things said about him, he's a proto-DLC-er who ran McGovern's campaign in 72, so he's likely on good terms with the "youth" vote; as long as he keeps it in his pants, he'll at least be the frontrunner going in
Glenn -- been in the Senate for about as long Hart, but less accomplished
qua Senator; his big leg up is his astronaut persona
Brown -- if, TTL, he can keep a lid on some of his weirder ideas,* while not losing his passion for space and the environment, he'll be quite formidable -- a lot more formidable than he seems to be getting credit for, definitely more so than Glenn, and probably Carey
So I'd say, looking at this list, the real top tier is Gary Hart, Jerry Brown, and maybe Hugh Carey.
*(which butterflies may well help with -- having a shot at the Presidency can do that)