After being taunted with Mt. Silver's existence ever since I first saw the town map, I was happy to head to Victory Road to go check it out.
"You can't go there! There are scary-strong Pokémon in there," the guard said. I actually found out later that that was a big fat lie, the "Scary strong" Pokémon were actually at a perfect level for grinding before the Elite Four. Anyway, disgruntled, I beat the Elite Four and returned. Nope, I still had to beat the Gym Leaders of Kanto, it seemed.
A long time later, the gym leaders were beaten, and I went into the most underwhelming area in Pokémon since Ever Grande "City" (actually, EG"C" came later, but anyway). The Pokémon there were an insult to Cerulean Cave, the post-game area of RBY, but regardless, I explored the caves. I went deeper and deeper, higher and higher, until I entered a cavernous, yet well-lit room where a narrow path went across a huge abyss. At the other end stood a figure I knew all too well. Why was he here? What did he want? I talked to him, and he replied. Actually, he said nothing, but he replied. Then he grabbed a Poké Ball and nodded slowly. Slightly nervous, I did the same...
Thus began the battle of my childhood for the first time. It took me a few tries to get past him and his endless Full Restores, and that Snorlax. But that first encounter... definitely a favourite moment.
In second place, the feeling I got when I completed the Magnet Train sidequest and was able to take the train for the first time. It was as if everything suddenly fell into place, Kanto and Johto were connected once again and the entire world was just a short trip away, instead of that once-a-day boat ride or the trek via Tohjo Falls.