TL;DR I'm playing Assassin's Creed Unity this week.
I've been playing the entire Assassin's Creed franchise in release order and this week I'm at Unity. It was actually the first Assassin's Creed game I've ever owned, bought the collector's edition when it came out.
I first heard of and saw Assassin's Creed when Black Flag released a free demo on the Xbox One. I absolutely loved it, so the first game in the series to come out after that I just bought right away (and it was Unity). I played it a bunch, finished it and got most achievements, but it didn't really stay with me that much.
The next game in the series that drew my attention was Valhalla and when I saw it on sale for a ridiculously low price I just snatched it up. Absolutely loved that game, played it to 100% completion of everything, got the game pass and exhausted all content possible out of it.
I wanted more AC and grabbed Origins on sale. Loved that one too, played everything out of it. Still needed more.
But there was a lot of present-day back-story in Valhalla and Origins that I didn't know about, which is why I decided to go back to the beginning of the series and play every game in order. It has been a great journey, I love seeing how the series progressed, and I also now understand why Unity has such low reviews (every game so far has been a visible improvement over the previous one, except Unity which looks and plays worse than Black Flag). I'm still going to 100% it before moving on though (looks like back in 2014 I didn't go for 100% on the main sequence, so I scrapped my save file and started over, since I didn't remember the story anyway).