Which alien movie jumped the shark?

All the ones after Aliens, especially Alien 3.

I wouldn't consider Alien 3 as jumping the shark. The production was a mess and thus to have such a grounded and watchable film is testament to the ability of the director. I think it's probably the most fitting ending I could imagine. It's almost like the anti Star Wars, the middle is the high point for the characters whilst it's bookended by endings of bitter sweet.

Resurrection is where it jumped the shark, but I really don't mind it. Resurrection has some extremely interesting ideas and it is very much a product of its time, being an over the top, almost satirical look at the series it stems from, an entertaining movie of the 90's all around.

When you start getting into AVP and the prequels is when the series becomes objectively poor or simply convoluted, missing what made the originals great whilst desperately trying to stay relevant. They simply feel more produced.