You mean the moments of 'soft reboot', which are a good jumping on point for new viewers?
Spearhead from Space, yes, definitely - not just a new Doctor but a new Earthbound setting, plus the switch from black-and-white to colour (although we'd met the Brigadier, Sergeant Benton and UNIT before). The Classic series didn't have showrunners in the modern style, the show back then was 'run' by a head producer and script editor as a team, and the changing of that production team could at times be classed as a soft reboot for the show. Which gives you Robot/Ark in Space as another soft reboot moment - and I list both because although Robot introduces a new Doctor and a new companion and is the first serial under a new production team, it is also a holdover from the old UNIT era, whereas Ark in Space sees the new Doctor leaving UNIT behind and striking out into the wider universe once more, and with it being Harry's first trip in the TARDIS it can work as a soft reboot story to introduce the show to new viewers, despite also having Sarah Jane, who had been around for a couple of seasons already. Actually, the Fourth Doctor's era has a few good soft reboot moments for new viewers to jump aboard - such as The Face of Evil, which sees the Doctor travelling solo and meeting a new companion in Leela, who has a bit of an Eliza Doolittle-type character arc over the next few adventures. He is travelling solo again at the start of the Key to Time series (season 16), which gives us a new companion in Romana and has an ongoing storyline that follows through the entire season. I would count both of those as soft reboot moments under your definition above.
You're counting An Unearthly Child? That isn't a soft reboot, it's the origin story of the entire show! After that, the First Doctor's companions always overlap. The Second Doctor's first adventure is not a good soft reboot point, as there are companions already with the Doctor and the story itself I think would be confusing for new viewers (plus it exists only as audio and animation). The Fifth Doctor's first adventure is Castrovalva, but again he already has a team of companions with him - although two out of three are brand new, and although the story its a bit abstract, I think I would count it as a decent soft reboot point for new viewers to jump on board. It feels very much like the start of a new era and has a very different tone to what came before.
The Sixth Doctor...well, he has an existing companion with him when he regenerates, and The Twin Dilemma is awful, so not a good soft reboot point. And then Time and the Rani introduces the Seventh Doctor, but again he has an existing companion with him - and spends much of the adventure with the Rani who has disguised herself as Mel, so...probably not a great one to watch as a soft reboot! Dragonfire would probably make a better soft reboot point for the Seventh Doctor's era, as it introduces Ace to take over from Mel and feels very much like a fresh new start.
Does that help?