Essential X-Men, Vol. 1 contains Giant Size X-Men #1 and X-Men #94-119.
The X-Men battle Krakoa, Count Nefaria, Kierrok the Damned, Eric the Red, The Sentinels, Black Tom Cassidy, The Juggernaut, Magneto, Firelord, The Shi'ar Royal Guard, Alpha Flight, Sauron, Warhawk, Mesmero, and Moses Magnum. Thunderbird dies. The Phoenix Force is awakened.
Uncanny X-Men was all reprints of earlier X-Men comics when Giant Size X-Men came out featuring a new team of X-Men, led by Cyclops, to rescue the old X-Men. The next thing you know, The X-Men suddenly become Marvel's A-List players. The multi-national roster of X-Men, penned by Chris Claremont and drawn by John Byrne, was THE comic of its day.
While I wouldn't say it stands the test of time as well as some books, it's easy to see why this book was the dog's bollocks back in the day. So much crazy shit happens! After Len Wein and Dave Cockrum got the ball rolling, Claremont and Byrne took the ball and kicked it into the stratosphere.
Essential X-Men lays the groundwork for the X-Men being Marvel's top franchise for a couple decades afterwords. Jean Grey becomes the Phoenix. Wolverine, Colossus, Storm, and Nightcrawler join the team. We'll just gloss over Banshee at the moment. Magneto re-establishes himself as the top villain. The Juggernaut and Sauron are brought back into the fold. Muir Island is established. Professor X and Lilandra of the Shi'Ar begin their relationship.
For an iconic X-Men read, this is it. This is where the X-Men join the A-list, where they break out of the pack and become Marvel's meal ticket for decades to come. Four out of five stars.