The Christopher Eccleston, David Tennant, or Peter Capaldi.
In the case of the New Year Special, "Resolution," Chris Chibnall's script contained a single scene that tied in cleverly to the classic series. Fearing the threat of a Dalek on Earth, the Doctor called for help - the first time she's ever needed to since her regeneration, an indication of the scale of the problem facing the planet should a Dalek stay loose for long. She put out a call to U.N.I.T., the Unified Intelligence Taskforce, an organization that various Doctors have worked alongside since the Patrick Troughton era.
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Unfortunately for the Doctor, it turned out U.N.I.T. no longer exists. The organization has been suspended, and it looks as though there are two simple reasons. The first appears to be austerity, with mention of a spending review; in the United Kingdom, over the last few years public spending has been slashed for many functions of government, and between 2010 and 2015 even the armed forces budget was cut by £8 billion in real . Critics of the Government's austerity policies argue that even essential services have been cut, and in the world of Doctor Who it seems the Government completely withdrew funding for U.N.I.T..
A second factor also seems to have played into this. In Doctor Who lore, U.N.I.T. was conceived sometime in the 1970s, and was founded under the auspices of the United Nations. In 2019, of course, multilateral ideas like the United Nations or the European Union are out of fashion - particularly in the U.K. and the United States. It sounds as though the British Government wasn't the only one to abandon U.N.I.T., and the organization is now completely defunct.
This leaves Earth extremely vulnerable. U.N.I.T. had built up a vast amount of experience, dealing with everything from Daleks to Daemons. They'd developed specialist resources for taking on alien presences on Earth; by the Seventh Doctor story "Battlefield," they had silver bullets for werewolves, gold-tipped for Cybermen, high explosive rounds for Yeti, armor piercing for robots, and even teflon anti-Dalek shells. By the Tennant era, U.N.I.T. had their own mobile Helicarrier base, which they used to curb a Sontaran invasion. All that knowledge, and all those specialist resources, are no longer available in the defense of humanity.
It's a smart move for Doctor Who showrunner Chris Chibnall. It means that Earth is now protected solely by the Doctor and her "extended fam," with no additional backup or to help them. The stakes are thus infinitely higher going forward into Doctor Who's season 12 more classic monsters will return, and the Doctor won't have any help dealing with them.