This story absolutely deserves all the praise it gets and then some.

An Unearthly Child
If I could rank this separately it would be an easy 10/10. Ian and Barbara are instantly compelling and likeable, and the mystery of what's going on with Susan does a brilliant job of grabbing the viewer.

The story is also remarkably paced considering other stories of the era, I was especially surprised at the flashes to Susan in class when Ian and Barbara, knowing how stories at the time were filmed I just didn't expect those quicker cutaways but they're there and work brilliantly at adding to the mystery.

The atmosphere from when Ian and Barbara are waiting at the junkyard is spectacular, the black and white colouring and darker lighting adding to the sense of mystery, and generally everything in that scene building and building to that moment when they enter the Tardis is incredibly done. The contrast between the dark and dingy junkyard and the prim and pristine Tardis interior is marvelous.

Then that final shot, the Tardis in a mysterious new place with a humanoid shadow looming into view, it's great.

100,000 BC
While definitely the weaker of the two parts of the story, 100,000 BC is still really good, and holds up impressively well all this time later.

I've seen a lot of people complain about the caveman politics, but honestly I think it's really compelling. Our characters are thrown into the middle of a conflict, taken away from the Tardis, and need to figure out what side to assist in order to get back (setting the groundwork for plenty of stories to come, though it surprised me with its visceral brutality.).

What works especially well for me here is the unique position our core cast find themselves in here, being from the future and having the ability to create fire they have an advantage, but being disconnected from the goings on, distrusted by the tribe, they also have their own unique problems.

I live how Ian takes the lead, One trying to do so but not quite being trusted by Ian and Barbara yet. I like how when asked who their leader is though towards the end, Ian then points to One. I also really like the bit about how in their 'tribe' everyone can make fire.

Overall, An Unearthly Child is a brilliant story that perfectly brings people into the world of Doctor Who, shows what the rest of the show will be like and about, and perfectly utilises the show's time travel aspect, showing just how far we have (and haven't) come as people today.