I view the Packers and 49ers are more fierce rivals than the Giants. Seems more frequent that vs Packers or vs 49ers with something at stake occurs.
For the most part, Cowboys and Giants seem to alternate competitiveness. We've only met the Giants once in the postseason. Quick glance and both teams making the playoffs in the last 50 years has happened maybe 5 times.
Recently, 1993 Emmitt game, 2007 Div Round, 2011 Week 17 Do-or-Die are important games.
Just off top of my head:
Packers 7 playoff games: NFCC in Cotton Bowl with Don Meredith end zone INT, Ice Bowl, 1993 playoffs, 1994 playoffs, 1995 NFCC, 2014 Dez catch, 2016 Div
49ers 6 NFCC: NFCC 1970, NFCC 1971, The Catch, 1992 NFCC, 1993 NFCC, 1994 NFCC
There's an NFL Films documentary dedicated to Dallas vs SF.