There will be a few more coming in, notably Biden. But fundraising is going to be a big determining factor and I expect half the candidates will not make it to Iowa.
Taking into account that I expect Biden to jump in I place the top 5 at:
Biden and Bernie are getting up there, both would compete with Trump in debates with Biden having a decisive edge in the middle of the country.
But Beto plays well in the middle as well and has a major advantage over Biden in fund raising. Biden needs large donors, he doesn’t have the small donor appeal Beto and Bernie have.
Warren is a strong candidate but flawed, the DNA test was a terrible idea. She is constantly playing defense and that means Trump can get under her skin, which would be bad.
Booker is strong as well, but he’s gained so little traction or buzz since he announced. That Mayor from South Bend has gotten better press at this point.
The old saying goes “Democrats fall in love, Republicans fall in line”. If that is still the case, Beto is going to be hard to beat. Plus, seeing a vital and attractive candidate paired off against Lord Cheeto is going to be terrible optics for Trump’s campaign. He would much rather run against someone his age.