The original VN is always the go-to source of everything fate and where a lot of the concepts and caharacters of the franchise started.
There are two versions:
- The original that includes the sex scenes.
- The Realta Nua version that includes Voice Acting and other goodies.
If you’re son inclined there are versions that include everything.
Fate doesn’t have a MAIN timeline, concepts are shared in most timelines but characters can differ quite a bit between titles. For example, the Extraverse is a world where magic mostly died and humanity started looking for the Moon Cell, a Hyper Advanced Super Compuer left by an unknown alien race for a way to stop stagnation.