Engine? Database? GUI? Repertoire?
The best chess program.

software is oldschool.
The planet is yours when we pass on. But, as long as I'm breathing such things as books and software will find a market. I use lots of software (ChessBase still puts out the best for chess), and I'm helping find the bugs and errors in chess mentor (within the limits of my subscription level), but a book and board on a table in a cafe remains one of my preferred ways to study chess. My hair is gray and I live by one of those pill boxes with my daily dose from a vast pharmakon, but I can still whip most of the younger set in head to head competition (and I'm getting stronger).

I believe that the Shredder Linux version is not exactly the same as the windows version....

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For all that it offers, the Chessbase application with any decent engine plugged in.
Any improving player's happy place usually involves a game window nicely organized next to an engine analysis pane, an opening book tree and a move-by-move scan of a 4+ million game database. Nobody does this better than chessbase!
What is the best chess program overall? It can be on PC,MAC,DS,(any platform.)