If I had to guess who will be the nominee, I'd have to guess Kamala Harris. She's immensely popular and doesn't have much national political experience. She may suffer a bit in the primaries due to her very tough on crime views while serving as DA of SF and then AG of CA, but she'll likely survive that. Her limited time in the Senate allows her to keep her hands mostly clean and the last primary and general election on both sides showed having a history can make life awfully difficult.
Second guess would be Hickenlooper trying to run with the whole moderate schtick. I don't think it'll work with everyone so polarized and fired up, but we'll see.
Dark horse: Pete Buttgieg. I think he'll run and lose. He'll parlay that either into a successful Governor or Congressional run in 2022 or potentially be a VP pick.
Darker horse: No idea if he has any desire to do so, but Schiff has made quite a big name for himself and could be the golden boy based on how these investigations go.
As for who I want it to be, I want a Democrat who's damn good on foreign policy, especially wrt to China and Russia (or at the very least someone who recognizes how important FP is and gets an elite staff). Trump has made countering China in Asia and the rest of the world so much harder than it had to be. We need to put some serious work into rebuilding alliances across the entire world if we are to limit China's growth and ensure our continued place at the top defining the rules and standards of international politics, trade, and technology. Being able to do so has brought us an incredible ROI and we need to keep that power. As for Russia, their economy is in deep trouble and Putin's wars and military parades can only distract from the fact for so long. Obama's sanctions during a time of increasing oil prices starting a run on the ruble was very advantageous for us as it resulted in Russia burning through its forex reserves to stabilize the currency and an overall weakening of their economic situation(read more here as they have a surprising good summary of it: https://en.wikipedia.org/wiki/Russian_financial_crisis_(2014–2017)). GDP growth has been terrible with an absolute collapse from 2014 (start of the annexation of Crimea) to 2017. Having a massive loss in GDP during a global recover/boom is terrible and now still having a 1.5% growth rate as a developing nation is terrible. Economic and geopolitical forecasters warn about the political situation as a result of this come 2023-2026 and it'd be great if we had a foreign policy team that could take advantage of that.