The year I remember most fondly is undoubtedly 2003:
Metroid Prime (2003 in Europe)
Wind Waker (2003 in Europe)
I'll never forget the year as the year I sold my launch PS2 with about 15 games. I was tired of the games released and had already played MGS2 and GT3 to death. I bought GTA3 due to the incredible hype and was quite disappointed by how quickly I got bored.
I thought Gamecube wouldn't be released in South Africa, so when I saw that awesome purple box in my VG store for a very reasonable price and concluded that I must somehow buy it. A week later my PS2 was sold for a healthy price and I got:
Metroid Prime Gamecube bundle + Resident Evil Remake + Starfox Adventures.
Each of these games shat on anything the PS2 could do visually, and I was truly enamoured by the console's small discs, fantastic controller and awesome graphics.
Shortly after I got Wind Waker (aah, that golden box art) and playing through those games was an absolute joy.
I don't think anything will ever beat falling in love with Nintendo's awesome console. I never, ever regretted selling my PS2 for the Gamecube experience. So 2003 was for me the year that made Nintendo #1 for me again.
Also, I highly doubt whether anything will eclipse the experience of playing Metroid:Prime for the first time. It was truly an earth-shatteringly beautiful experience.