Probably one of the most interesting stealth action game I have played, vastly prefer it over the batman games. I will probably write a full review someday detailing my thoughts in a more cohesive way.

Something that Assassin's Creed perfectly understand (that its later entries as well as games that inspired failed to capture) is its ability to bring sense and meaning within its own mechanism all while being progressive towards stealth as a genre.
What I mean by that is that, Assassin's creed 1 goes all in authenticity by viewing the conflict of the crusade from a muslim point of view. Part of that perception is put on full display in the way Crusaders are presented to you in-game. Europeans had little interest for Islamic writters something that's actually shown in the game notably in one passage where books are shown being burnt which also ties up to the theme of knowledge and illusions.
Navigation here is understanding the philosophy of the Assassiyiuns, we live with them and try to understand this complex political situation as well as the morality of our own actions which is explored through Altaïr's growth as a character.

Assassin's Creed is a game that sets itself apart from the rest of stealth games by forcing players to be stealthily in social situations, much like how Isma'ilism, the order of assassins that inspired the creed, used to operate. It's all about carefully planning an assassination as well as memorizing an escape route. The game will often varry its missions, sometime even subvert its own structure, one mission in particular has you be the target instead of the other way around.

Anyway, I don't really understand the low rating this game has received, for me it was an incredible well thought out game. As I've said it above, the later entries fail to capture what was so special about it. Though I've only played Unity and a bit of AC2 and I refuse to touch the RPG ones.

Edit : Social stealth games have existed prior to Assassin's Creed (notably Hitman), the way it encourages rotation between stealth and action and ties it to an open world is what makes it unique. Switching between low profile, high profile and how this affect the way the population moves. Altair will topple over if he bumps into people in high profile, so really during those intense chase, the population becomes an active obstacle you must avoid, forcing you to rely on the parkour system.