A few different categories for me.
As a Saints fan - Saints v Roosters Anzac Day this year. It was close, high quality, awesome atmosphere, and got the dragons out of the early season slump. Also, Saints v Bulldogs in 2001 semi-final at Homebush Showground, Saints winning by a point.
Highest quality game - Warriors v Broncos at Ericsson Stadium in 2003. It had the lot, brilliant attack, smashing defence (Villisanti & Meli made some of the biggest hits in recent times), and a brawl to boot. It had it all.
Sheer enjoyment - Cowboys v Roosters semifinal last year. Cowbangers just fell short, but played their guts out and the Roosters had to play at their best to beat them. If they'd have won it would have been one of the best games of all time.
It sh*ts me how often the Channel 9 commentators will label a game, "the best game I have seen" so often.