Given what I know about them and the potential attacks they face in a general, I can't see Warren, Harris or Sanders winning against Trump. I think Biden would be a tossup as he could get some of the "white guys in the midwest/south who don't like Trump" vote - but he wouldn't be as strong a candidate. Booker is the one that intrigues me - I think he would have a legit shot of rallying the younger voters. Outside of that, I just can't see any of these people winning. Then again, I didn't think Trump would win the republican nomination (and the general) - so take all this with a pound of sale
The last thing I will say - republicans vote on "fear of the opponent", democrats vote on "how inspiring their guy/girl is". So a bad nominee doesn't hurt republicans nearly as much as democrats - especially if a someone that can be branded a clear republican villain like Warren, Sanders or Harris is nominated.