Actually my real fondest moment of Halo and for any game at all is this.
While 6 years old
Starting out, me and my brother (from a another mother) only played offline on Halo CE, spend hours doing this, days ! Then Halo 2 came out and playing through the campaign continually and in CG, just me and him versus each other, nobody else (did not have internet connection back then), he would eventually figure out how to do the rocket lock on and would not tell me, man that annoyed me, but non the less then Halo 3 came out and this is where things escalated a bit.
Never before have we played Halo online but now with Halo 3 (which I bought my own console for) we could suddenly play online, we would still go to each others houses and play, but this new experience with multiplayer led to something completely new, we met others from our rather small country (denmark) that also played Halo and we then grew our friendships until one day we decided to have a lan party. From all over our country we came, 9 of us in total, and we played, had good times and ever since then we have always met up again and kept the contact. This is my fondest "memory" of Halo, being the game that gave me friends for life.