The first two generations of the Pokemon games (namely Yellow, Silver and Crystal), and Majora's Mask.
Yellow was the first game I ever owned and I remember being amazed at how such a huge world could fit on such a tiny cartridge. My whole world was Pokemon back then and I was constantly immersed in it. When the second generation came out my mind was completely blown at the huge step up every aspect of the game got. It was like my dreams came true. The ability to go back to Kanto after finishing the Elite 4 was great as well. I could go back to all the places I went before and see how they changed. It was like two entire massive RPGs in one game! I was even more amazed how they could do that on one little cartridge.
Because of Pokemon I got myself an N64 so I could play Pokemon Stadium, and Pokemon Snap. One day in a game shop I came across a strange looking box with a crazy looking mask on it. I took it home and found another huge magical world to fall in love with. I guess I'm unusual in MM being my first ever Zelda game. I had no idea the series even existed before I got MM, so I didn't realise how different MM was for a Zelda game. I just played it and loved it. The world in this game may not have been as big as in Pokemon, but everything about it was so detailed. I got really connected to all the people and their own stories, particularly Anju and Kafei, although I would never complete their side quest, or the game for that matter, until much later when I got the game again on GC. Yet I still loved that game as much as ever.