I definitely think O'Malley. He is going to try and run as soon as you can so he can build up on his name recognition and be more of an established candidate when the rest of the field announces.
O'Malley's an interesting case, because I think he could actually get a tangible benefit from announcing early, which stems from the fact that he already has a decent level of name recognition in Iowa (at least compared to most of the field) from his 2016 run, and yet he's not really covered or even included in most of the polls right now, as the Senators are getting much more attention from the national media.
I don't actually believe that
Iowa poll by O'Malley's PAC, which didn't even include Warren as an option. However, in a fair poll of Iowa, with a realistic selection of candidates who might plausibly run, O'Malley making it into the top four on the basis of name recognition at least certainly seems *possible*, but that only works if you include O'Malley as an option in the polls, and most other polls so far don't. So the benefit of announcing early would be that the media would notice him enough to at least include him in the polls, and then maybe just by showing up at 3rd or 4th place in a couple of polls in Iowa, the media starts talking about him, and he builds from there. Though I guess this only works if candidates like Gillibrand and Harris don't grow their own name recognition at all between now and November 2018, or whenever O'Malley wants to get started.