I have played A LOT of N64 games in my past, and I still have the cartridges from years ago. When I count them today I have twenty-three! However, the games I remember most fondly are Mario Party 2, Mario Kart 64 and Goldeneye 007, and if I were to narrow it down to just one, the definite answer would be Mario Party 2.
I LOVED having my friends and family play an everlasting round of Mario Party 2 with me. I loved the idea of a Mario party-type game, and since my dad wouldn't let me get the first Mario Party, my childish self went out with a few friends about two years later at the launch of Mario Party 2 and we played it for the whole rest of the night until about six in the morning. And I was 14! Sadly, I lost my Mario Party 2 cartridge in '06, a day I will dread for the rest of my life. I can't count the endless amounts of minutes I was poking my head under tables, IN tables, sneaking into other's privacy and looking at friend's houses.
Luckily, just recently, I found exactly the game I was looking for on the Wii Shop channel for ten bucks - Mario Party 2. So for endless amounts of hours I play the game I haven't played in seven whole years. So, yes, Mario Party 2 may just be the best game ever on my list, even though I'm way into the mature genre.