Mario's Tennis isn't the launch for VB! Mario Clash was!
Anyway, I'd have to say Super Mario 64. It came out when I was 8, so I used to sneak up town (cause I wasn't allowed) and play the sample version at a store for hours on end. Although I don't have N64, I play it at my friend's house all the time.
Another one of my faves was SMBDX. This game was awsome! It had so much! Unlike the 2 game SMA, Super Mario Bros. Deluxe was full of stuff. The original, the challenge mode, the VS game, then you could play Lost Levels and even Race Big Boo. Also there was a photo album and toybox which just added to the excitement, and it was so full of stuff, it was great~!