The first 10 issues of Uncanny X-Men really feel like the foundation era of the series.

The first 10 issues of Uncanny X-Men really feel like the foundation era of the series. This is where Xavier's school is established, the original five are put into place, Magneto becomes the central threat, and the Brotherhood starts taking shape. You also get those early repeated themes that would define the title for years after that: team combat, moral conflict, manipulation, outsider status, and simpler but focused storylines that still worked really well. These comics may be early, but they do a lot of the heavy lifting for what X-Men would become.