It's only because of it stopped the perfect season, it's not because the game was actually that good. I agree super bowl 38 was the best super ever. If they put records aside and all and it went right down to what was the best game 42 wouldn't be that high.
I think similarly many considered (or still consider) SB3 to be the best/greatest ever. It may have been the biggest from a historical perspective as it showed people for the first time that the AFL was just as good as the NFL; that game was rather boring and not really that well played. On a side note many incorrectly credit that game with forcing the merger; in reality the merger had already been agreed upon thanks to Al Davis.
Without much thought, here's my top ten:
SB23 (1988 season): 49ers 20, Bengals 16
SB36 (2001 season): Pats 20, Rams 17
SB25 (1990 season): Giants 20, Bills 19
SB13 (1978 season): Steelers 35, Cowboys 31
SB38 (2003 season): Pats 32, Panthers 29
SB32 (1997 season): Broncos 31, Packers 24
SB34 (1999 season): Rams 23, Titans 16
SB10 (1975 season): Steelers 21, Cowboys 17
SB43 (2008 season): Steelers 27, Cardinals 23
SB30 (1995 season): Cowboys 27, Steelers 17