Various favorites, for various reasons-
Sojourn/Toril MUD-
The first Manscorpion Hive raid and finding the Reptillian Rampager.
The 12+ hour CR from Roots with Mikar. So many people involved, so much gear on the ground from rotted corpses, so many sets of backup gear being compiled and used, so much nervousness about whether the game would crash and we'd lose it all.
Related- sneaking down to Undermountain 2 to trigger a crash bug to save a group who wiped in the crypt after taking the key in with them before their corpses rotted, back when Undermountain was still an unmapped terror to almost everyone. And appreciation for the baby assassin that was willing to risk coming with me to pick the door to UM2.
EQ-
First time entering South Ro, and the vista of desert stretching out before me.
Soliani (an FoH bard) giving me a glowing stone band- bestest gift ever! Thank you again, wherever you are nowadays.
All the work and organization it took to get into Time.
All the people, nearly half of which were FoH members (I wasnt in FoH) coming together on the spur of the moment to whack Ragefire for my epic with less than an hour before server downtime.
Pools of Radiance- the eleventy-billion kobold attack.
Red Storm Rising- doing the game with the oldest, least capable sub in the game and spanking it.

Baldurs Gate- Lots of stuff of course, but in terms of emotion for some reason the opening scenario of breaking out of captivity with Imoen.
Star Control- Finding out why the Urquan did what they did.
Thief- sneak sneak sneak sneak...
Rome- Total War, playing with Greek phalanxes.