Might be Robinson Crusoe here, but I have to go with live games rather than watched on TV games, so 1991 QF, SF and Final RWC, 1999 SF and Final miss out.
3 games I was at that were amazing for different reasons.
Sydney v Wales - Sydney Sports Ground 1978 - Laurie Monaghan nailed it with a droppie from near 50m at the death for a 2 point win. My first big international game live, and Wales then were stars.
Wallabies v All Blacks - Olympic Stadium 1999 - 109 K and we won 28-7, still the largest winning margin ever against them. Great atmosphere and a completely dominant performance. Best and loudest anthem I ever heard. Kafer at 10 too!
Waratahs v Crusaders - ANZ Stadium 2014 Super Rugby Final - great atmosphere, nail-biter but so sweet when the droning, boorish Kiwi fan sitting nearby finally had to suck it up at the final kick, having spent the whole game bagging all things Australian. Funnily wasn't even in Crusaders gear!