Ghost took inspiration from AC, that much is sure. But the world, mechanics and combat system they crafted for that game, as well as the style of gameplay is far better than any AC game IMO. Ubi worlds somehow all manage to look and feel like the same thing despite their different settings. I just couldn't get into Valhalla as much as I wanted to. The combat felt weak and didn't flow. The map and story was bloated and confusing. I never quite knew where I had to go or what I was supposed to be doing. AC is also saddled by its 'world'. The animus and Sci-Fi Alien connection, and the 'Assassins' v Templar stuff, hidden blade etc. hold it back IMO.
Ghost is a game that captured me from the beginning. They did what no other developer had done for a long time and gave us a grounded, semi-historically accurate samurai game. Not some anime inspired, over the top, campy blonde wigged JRPG. Not some supernatural ghouls and monsters. They built a world that I had been longing for in a 3rd person action game since Shogun: Total War, Kengo and Way of the Samurai in the early noughties. It was like being in a Kurosawa flick or one of the many history books I've read and every second of it was amazing. The story had it weak points overall. The 'samurai to ghost' journey and over the top depiction of Samurai honour vs the 'dishonourable' way of killing Mongols I feel was a bit unbelievable, but it was enjoyable. I only wished they dialled up the realism even more, but it was fantastic. The gameplay and combat was also smooth as silk and far more satisfying than anything in any AC game I've played in recent times. A bit more budget and polish with some better physics/realism, I feel Sucker Punch can make a GOAT 3rd person open world game.
EDIT: I should add, Ghost was my favourite game on PS4, and top 3 of all time. Love it.