What is the best chess database out there (quality vs. price). Do you recommend online or offline chess database. Any experience with chessbase, chessok, chesstempo or openingmaster?
The TWIC (This Week In Chess) database is your best bet. It's free, has millions of games (many of which are recent and up to date), and is of a high quality.
Here's the link: http://theweekinchess.com/twic
When you get there, scroll down to the "TWIC Downloads" table and download all the PGN or all the CBV files.
After downloading everything, merge all the games into one database in ChessBase.
Then, you can make this database your reference database (if you don't know how to do this, look up a YouTube tutorial on ChessBase's official channel).