Dark Souls: First playthrough, getting murdered in the graveyard/catacombs then strolling on down to Valley of the Drakes and running into the ogres near Blight Town and running my ass off as they chased me back to Firelink.
Darklands: Taking down my first Raubritter by fighting his army and then looting his entire force. Using the money from this venture to increase my parties alchemist and priests knowledge.
Ultima 5: The whole Lord Blackthorne executing party members. Realizing the mantra's to the shrines hadn't changed since Ultima 4, and using my notes from that game on this one. Also, note taking, lol, wtf is up with that right?
Morrowind: Finding awesome magical items outside of scripted events/quests. Becoming "immune" to the plague because of an experimental medication. Realizing the game opens with you getting out of prison only to crash at a druggies house (look under Cauis bed
) So much for halfway houses, eh Emperor? Vivec and Almalexia; really damn interesting npcs.
Daggerfall: All the political intrigue and factions! The dungeons. Telling quest givers to shove their "Find the bauble" dungeon quests up their asses. Pissing off npcs and then murdering them. Taking out exorbitant loans from various provinces and then gtfo-ing.
Ultima Underworld: Itemization and puzzles. Combat wasn't bad either for what it was. Ruuuuuuuuune magic!
New Vegas: Dead Money, Old World Blues, etc. Some of the best expansions for a game I've ever seen, Bethesda should have been shitting themselves in shame. Also the base game, damn good stuff, C&C at it's finest. Minus that whole bit about the Powder Gangers, and the quest to push them out being tied to having good relations with them at some point. Bleh, whatever.
Skyrim: Dem water textures!!!