The 'Alien' saga is a bit of a tangled web if you dive in without a map! First, there's 'Prometheus' (2012), which serves as a prequel, exploring the origins of the Engineers and the xenomorphs. Then comes 'Alien: Covenant' (2017), bridging 'Prometheus' to the original 'Alien' (1979). After that, it's 'Aliens' (1986), 'Alien 3' (1992), and 'Alien: Resurrection' (1997). The crossover films 'AVP: Alien vs. Predator' (2004) and 'AVP: Requiem' (2007) exist in their own weird corner of the timeline, but they’re not considered core canon by most fans.
Personally, I love how 'Prometheus' and 'Covenant' add layers to the mythos, even if they split the fandom. Watching them in release order first gives you that classic horror-to-action shift, but chronological order is a wild ride for a rewatch. The later films... well, let’s just say they’re an acquired taste!