I don't generally like sf, film or otherwise, but try "Primer". Best movie ever made for <$6000 AND arguably best sf movie.

I find it difficult to approve of a time-travel movie whose basic premise is causality violation.

In that sense, Bill and Ted's Excellent Adventure was a better sci-fi movie than Primer. Far less thought-provoking, true, but making a simple-but-unparadoxical plot is a greater accomplishment than a complex-but-paradoxical one.