Starting Ragnarok Online for the first time at a friends request. And then subsequently exploring every area in the game. I started as a Mage, and my friend told me to focus on getting Firewall before anything else. I remember leveling with him and watching as my cast time and damage rose as quickly as they did, and I got hooked. And every new place I visited was filled with such splendid scenery. From ice caverns to haunted clock towers, and so on. The skill system was fantastic. I really loved that game.
Now I'm building new memories in FFXIV. Leveling doesn't feel quite as great as it did in RO, but aside from that it's a definite joy to be had, and I'm having a blast playing.