I have the fondest memories of AC2 and it’s sequels. I’d say if you went there you’d be best to play those in sequence, but they are also the oldest of the ones that come recommended so likely will be harder to engage with that this point. Certainly, when I decided to play AC2 relatively recently, there was some tweaking to be done to get the controls to work. They were good at the time, but hard to know whether it’s work starting afresh there now.
As a relatively modern standalone experience bringing the original Assassin’s Creed style, I think Black Flag is the one to probably go for, if you choose to at some point. Edward Conway is an interesting protagonist for an AC game, the pirate stuff is fun and it’s water vibrancy and sea-shanty’s were really fun. Saying that, Syndicate isn’t half bad, and I’ll also stand up for Unity now as well after some time and patching - I think it’s Paris is probably my favourite AC city out of the bunch.
The newer ACs perform a kind of paradigm shift at Origin’s focusing more on stats than the previous versions, which mark a bit of a change. The originals did have some stats, but it feel like it was more of a stealth focused experience about getting to your targets making the hit and escaping. Hard targets still went down like a sack of the proverbial when you stuck the knife in. Origins felt much more direct combat focused to me, and level focused. You hit a levelled guy with your hidden blade, he’s more often likely to pull it out and come at you again. Not saying that’s worse, but it’s a bit of different flavour. I assume it’s the same deal for Odyssey and Valhalla, but I haven’t played either of those yet, so others may have better opinions there.