Factor Five (Turrican series) would have been my vote.
I always liked the Core Design games. They seemed to put a lot more effort in their products than most software companies.
The Bitmap Brothers made some good games but they were all essentially Atari ST ports with marginally better graphics and music. I would have liked to see what they could have done if they developed primarily on the Amiga then downgraded the software for the ST rather than developing on the ST and porting to the Amiga with only minor changes.
It's hard to vote for a company like Ocean that seemed to just publish anything they thought would make lots of money regardless of the quality.