UNIX, of course...
Or, perhaps this is just ancient history?
These days, I use a combination of MacOS and NetBSD (a free Berkeley UNIX that runs on 22 different platforms! No hardware lock-in for me!), because that works well for me. MacOS is a pleasure to use (except when it crashes), and UNIX is really easy to program to do whatever data manipulations I want (and it's always as fast as the hardware I put under it - a very low overhead OS).
[Apple] [Cray] [DEC] [Sun] [SGI]