Despite the limited budget for a more realistic costume I really thought the Excalbians from "The Savage Curtain" were really alien.
Or you take the Shadows of Babylon 5
That is that their thinking and understanding was very different from ours, while usually Trek aliens are rather "mirrors" of ourselves and our ethnic variations.
What was really great about Solaris
(not the George Clooney travesty but the original Russian film) and a bigger theme in Stanislaw Lem's story was that the ocean intelligence covering the planet was so alien, that human science couldn't find a way to establish communication.