Aye, I'd forgotten about Sierra. Shame on me. Like Origin, they were major factors in the evolution of gaming. Judging Origin based on Ultima IX is fairly ludicrous. And let's not forget they did much more than Ultima alone. They were in no way a one trick pony.
LucasArts still exists, but has been but a shadow of its former self (yes, there is some rekindled hope on that front recently).
Ion Storm is a tricky one. While they'd be in a top 5 favorite list for me (Deus Ex may be my all time favorite game), like Troika, I don't know if I'd put them in my top 5 dead studios. I guess it depends on how you define the meaning of this list. I was defining it on how the company impacted gaming. While I adore Ion Storm and Troika, I don't feel they made the same impact as Origin or Looking Glass or Sierra. But perhaps that's unfair. Those older companies existed in a more formative time in gaming history, so I suppose it was inevitable they'd be changing the landscape of gaming. There WERE the landscape of gaming.
(Woah, got a whole nostagic wave when Lucky and Bedwyr mentioned Microzine and broderbund. Must...resist...temptation...to...break...out...][e emulator.)
Anyways, fun topic indeed.
Edit: SSI is worthy of mention as well.