In terms of atmosphere, for me as a Widnes fan, beating Australia in 1978. As a small child, I watched the whole game from pitchside over the boundary wall, as the crowd had spilled over from the terraces. Far more people in the ground than the official attendance. Must've been incredibly intimidating for the visitors playing in such conditions.
Obviously Widnes beating Wigan at Naughton Park to win the league in 1989, and Widnes beating Canberra at Old Trafford.
In terms of an epic struggle, hard to see past Wigan vs Manly in 1987 as mentioned above. Also GB's 8-4 win against Australia at Wembley in the first test in 1994 after Shaun Edwards was sent off.
My greatest memory from playing was probably not objectively a great game. We played a cup game in snowy, icy conditions at Runcorn and won by one try to nil, but it was one of those games where the whole team seemed to rise to the occasion and tackle their hearts out. Out of hundreds of games, it's the one that really sticks in my mind.