By far the largest subseries created by Type Moon - in both terms of popularity and number of enteries - the Fate series is almost certainly where you - the viewer - were introduced to the Nasuverse as a whole. Fate began life in Kiniko Nasu's highschool days, as an (as far as anyone is aware) incomplete novel that only got as far as the fight against Kojiro in what would become the Fate route of Fate/ Stay Night. This fate, nowadays referred to as "Old Fate" was very different from Fate as we know it today, with a female protagonist and a male version of Saber (unthinkable nowadays) as the lead servant.

Nasu would eventually drop Old Fate for years and would only pick back up the project after completing Tsukihime, turning his project into a Visual Novel and swapping the genders of the two leads. Fate/ Stay Night, as this novel would become known, was a massive success on its release in 2004 - leading to the release of multiple anime adaptations, a sequel named Fate/ Hollow Ataraxia and a prequel Fate/ Zero. This is of course, in addition to far too many spin-offs - resulting in this franchise being the overly stuffed monstorousity it is today.

For More on The First Three Fate Series:

While I am a big Fate fan, I'm going to be completely honest and saying that not all Fate Series are made equally. For every Fate/Stay Night there's a poorly plotted series Fate/Apocrypha that wastes so many interesting looking characters (Astolfo, Mordred, Amakusa Shirou) or a boring grindfest like F/GO (Although I've heard it get's good once you suffer through all the Singularities). Mediocre spin-offs simply come with the territory of being your studio's cash cow I suppose. In fact the first Spin-off outside of these three I mentioned (discounting Fighting Games) was, of all things, based off the Track Team Trio from F/SN and F/ha (at least according to https://www.tsukikan.com/tmdb/) - I've read the one transalted volume and it's as boring as it sounds. This is obviously a matter of taste, to a degree - Most people seem to like Koha-Ace and it's Fate spin-off more than me.

But, even so I find it difficult to care for much of Fate outside of these early titles and tend to avoid reading/watching/playing them. To a degree, it's like an extreme version of what's happened with Melty Blood, things got less and less interesting and the writing more self-referential as time went on.

I guess that's just what happens when you get to be such a big franchise. I should know, I used to get all my entertainment off DC comics.

Regardless of the issues that seem to be endemic to large franchises like this leading to flanderisation and exponentially growing barriers to entry, the Fate Series, just off premise alone is a classic - even in their bad anime adaptations - so you can only go so wrong when you have such a good formula to work off.