LaFollette. I'm a loyalist.
If we want to throw out Debs, we have to do it with a candidate who can attract as much cross-ideological support as possible. Ergo, La Follette.
I don't follow your logic. Why not Smith? He should be able to appeal to mainstream progressives as well as conservative Democrats, so long as they're not of the prohibitionist or bigoted variety. I'm tempted to vote for him myself, especially with Underwood, but I can't not vote for Cal.
Honestly I find all four choices very appealing.