It's like Far Cry but with plot and characters I care about, enemies that keep you on your toes, and a manageable number of collectibles.
I do like Far Cry but the writing and world building were never its strong suit. The characters' personalities are either non existent, or parody. The villains are usually the only ones I enjoy. Horizon has great characters with thought-out personalities, and I loved piecing together the mystery of why the world was the way it is.
And the weapons are much more fun. In Far Cry there are 4 slots and that's really all I needed (grenade launcher side arm, sniper rifle, chain gun, and bow and arrow). In Horizon you can use elemental damage to exploit a machine's weakness to make it explode and do AOE damage to its buddies, or turn them against each other, or hog tie a tough machine. I was constantly switching up my loadout depending on the challenge.
They have similarities but Horizon isn't just Far Cry with a paint job.