Percona's is probably the closest to the MySQL people are looking for; Percona is the premier MySQL consultancy, and they've patched the codebase for performance, etc.
From Monty's presentation about MariaDB, http://www.youtube.com/watch?v=AL1l9Puh0sk I got the impression that MariaDB is the primary open MySQL fork, with Percona merging their patches and becoming a downstream of MariaDB. Did MariaDB and Percona have a falling out in recent months?
