The best mode requirement still exists, although the America Invents Act largely removed any consequences for failing to disclose the inventor’s best mode.
The best mode requirement is one of the three separate description requirements found in 35 U.S.C. It states that the inventor must disclose any preferences relating to the claimed invention.
The goal of filing a patent application has to be to completely disclose your invention with as much detail and description as possible, paying particular attention to alternatives and variations.
The America Invents Act (AIA) has largely removed any consequences for failing to disclose the inventor’s best mode, which means the current state of the law is at best a bit murky.
Author's summary: Best mode requirement still exists in patent applications.