Robert Gentel wrote:
My choice would be The Andromeda Strain (1971).
As Crichton says in the book, we tend to anthropomorphize our aliens and make them bipeds, intelligent, etc even though statistically a less complex organism is more likely....
There can be no calculation of probability in the complete absence of data - but since we're speculating here, it seems to me that "aliens" if they exist would be far more complex entities than self-replicating crystals. "Contact" a book by Carl Sagan has a more plausible alien communications scenario: they're using radio signals with a mathematical sequence hidden deep into the digits of pi.
The sequence of events from sub-cellular life in the primeval ooze 4.5 billion years ago to us is so statistically implausible that I also don't see how aliens would end up being carbon-and-water based, let alone looking even remotely like us. Right here on earth, dolphins and whales have brains with more pre-frontal lobes than we do - and if intelligence increases with brain complexity that may make them more intelligent than us. Since we seem incapable of appreciating them I wonder how we'll spot any intelligent aliens we trip over unless they have the courtesy of introducing themselves.