I like the Dosadi books, Whipping Star, the short stories in Eye, Eyes of Heisenberg, Destination: Void, The Santaroga Barrier (which my wife hates), Under Pressure and Hellstrom's Hive. If I had to pick just one it might be Whipping Star but maybe Under Pressure is the hardest sci-fi.
Whipping Star has some amazing alien vs human discourse (at least, that's my memory from ~20 years ago!). It was the first time I found alien dialog that didn't sound like repackaged English.
