UPDATES - 24.11.2010
Just in. Sales figure is over 400,000
The following from this source
Masaharu Fukuyama's "THE BEST BANG!!" has topped the sales charts for this week, selling approximately 381,000 copies. This is now the highest first-week sales for an album by a solo artist this year, surpassing the 290,000 copies sold by Kana Nishino's "to LOVE." Fukuyama is also the first male solo artist since Ken Hirai in 2005 to have first-week album sales over 300,000.
Released on November 17, "THE BEST BANG!!" is Fukuyama's first best album since 1999, and it includes songs from throughout his entire career.
By Izumisano, on his previous sales record;
For comparison, his highest selling albums (reported sales):
1999 “Dear” MAGNUM COLLECTION 1999 (Compilation) - 1,387,351
1995 M Collection ～Kaze wo sagashiteru (Compilation) - 1,303,490
2001 f (Original) - 875,404
1993 Calling - 853,010
1994 ON AND ON - 712,010
If it is a single, then Sakurazaka takes the cake which I believe has a sales figure of over 2 million copies.