At the top of the list, because he's at the top of the polls, is Joe Biden. No one, or few, are really especially angry at Joe Biden -- it's pretty hard to be especially angry at Joe Biden -- but they look at some of his past positions and find fault with those, and with his inability to properly explain the context of them at the time.
What I've realized the last week is that for all those people supporting other candidates who are unhappy that Joe Biden is in the lead, every one of them should give thanks every day that Joe Biden is in the race. Even if he ends up getting the nomination (or not), they should be grateful.
The fact that Joe Biden is in the race and leading the Democratic polls is the reason why Trump has been obsessing on him and his son in Ukraine. Never mind that the issues were looked into long ago and no impropriety was found against the former Vice President, it is Trump's terror that he believes Biden is the strongest candidate against him that pushed him to his obsession of "being Trump" until his mob boss persona reared its ugly head and got him to shake down the president of Ukraine, leading to the whisteblower bringing this all to public attention and Nancy Pelosi finally allowing impeachment hearings to go forward and getting a majority of the House to support them.
Certainly Trump would have tried to take down whatever Democrat is his top opponent. But no other may have been perceived by Trump as being as likely to beat him (he'd have been wrong about that...), and none would have allowed for Trump to get involved with a foreign government to create this abuse of power situation, which seems likely to bring about his impeachment. Only Joe Biden.
Again, all studies have shown that Joe Biden did nothing wrong, and the timeline of what Trump is trying to suggest proves his charges to be a lie. But that's a separate matter. The point is that is Joe Biden being in the race and leading all Democrats is what ultimately has lead to Trump most probably being impeached.
And whether a Democrat support's Biden's candidacy or doesn't -- even if strongly doesn't -- should still be grateful every day that Joe Biden is in the race...and leading it. Whether they hope he loses or wins the nomination at this point forward is another issue entirely. But for right now, it is Joe Biden that has caused Trump to melt down and likely get himself impeached.