Both are excellent. Aliens is more enjoyable and watchable imo. But Alien is the better made film and will (and has) stand the test of time much better.
Aliens was already looking pretty dated with some of its action sequences and military equipment last time I watched it (like 10 years ago). In comparison, Alien has fewer action sequences, and its suspense-building thriller vibe is more timeless.
All pretty uncontroversial stuff. But when you start asking about the rest of the series that's where I start having unusual opinions:
- Alien 3 is the worst of all the Alien films by far, even including Covenant.
- Alien Resurrection was a great film. Much, much better than 3.
- Covenant was fine. The worst thing about it is that the negative reaction to it probably killed off the Alien film franchise for a decade or so.