The original Assassins Creed is usually dismissed as a "demo" or janky or some quaint, pathetic fore-runner to the actually good AC games.
While AC2 is my personal favorite game in the series, I love AC1 almost as much.
First of all the mechanics are almost identical to AC2. Yes, they improved it for the sequel, as you would expect, but parkour/movement in the original is amazing. Even the details like how you can actually control aspects of how the character vaults over small obstacles was there right in the beginning and the animations and "feel" are even better than in some of the later games.
One criticism is that the missions are repetitive. I laugh at this considering how much more "repetitive" pretty much all big games have gotten. There are folks out there playing Destiny and Genshin Impact every day who declare AC1's missions as being repetitive, like gtfo.
Yes, each of the nine main assassinations has the same core structure- you gotta walk around a district and do a couple of investigative missions before you can go after the target. I don't see why this is so bad, it pretty much is a common gameplay loop.
Some people complain about the flag collecting, which is also silly because you don't get anything for the flags and you don't have to do it and you shouldn't.
Some people complain about the grey/brown and that one is more fair. Still, the cities are different enough as to be thematically interesting and I still think they look great. We were coming out of the brown era for games so yeah for its time it looked great but I still think it looks great.
Most importantly, AC1 is the only game in the series that offers real immersion. You can turn off the HUD and navigate the game exclusively with environmental cues. In fact it was designed for that, supposedly they just added the HUD at the end to ensure players don't get too lost. Which, yes, you can say was as sign of the excessive hand-holding to come that many complain about (not me, though).
There is nothing like entering a district in one of the cities for the first time and just walking around. You look up to see eagles circling a tower and you know that is a viewpoint, so you climb up deliberately taking in how atmosphere changes up high. Sync the point, leap back down, and continue exploring. Overhear a conversation about delivering a message and you know that is a message to intercept and your first investigative mission. Do that until you find the Assassin's Bureau, or complete the investigation. Read the clues in your inventory for escape routes or more context, then go after your target for both a satisfying kill and continued exposition.
Yes, the annoying parts do get in the way sometimes. The infamous beggars and insane guys that hit you- I'm not gonna defend that shit. The citizen rescue bits that are just waves of fighting, I think that was stupid, really kills the whole shadowy assassins vibe. This I feel was the game getting in its own way and also looks ahead to the problems of Ubisoft bloat in the future. And the Kingdom area in between the cities was a big empty waste of nothing- it was supposed to be this big survival exploration extra thing but instead of just excising it when they didn't have time to include it, they left the area in with nothing to do in but just run in between the cities once each time so that you can unlock fast travel later.
But those weaknesses do not overwhelm the game's strength and promise for what the series could have been.
Oh, you know what else the game was actually good at?- the story! The whole meta-narrative with the animus, the assassins vs templars, the conspiracies- it was actually cool and promising. Sure walking slowly to read emails to deliver some of it was lame, but the content and promise of more was there and I remember how excited I was for it in those first two games. Now I just want them to remove any "modern day" from these new games 'cause man did they start losing the plot real quick with Brotherhood.