No definitive public record exists. Reliable annual sales data for board games in 1967 was not systematically tracked or published.
Twister had a massive 1967 spike (Johnny Carson + Eva Gabor on Tonight Show May 1966 → cultural phenomenon). It may have outsold Monopoly in units that specific year, but Monopoly's revenue (higher price) likely led.
Without audited data, it's speculation. Monopoly remains the all-time cumulative leader.