Oblivion & Fallout 3. Adore them both, probably won't ever reinstall 'em.
Oblivion was my first Elder Scrolls title, and it just sucked me in. Sure, now that I've got some more references, it's got many glaring flaws, but I still think rather highly of it. I mean, I had fun with it. Good times...
Similar story with Fallout 3, it was my first proper entry into the Fallout-series. I had played a bit of Fallout 2. I liked the idea, but it just wasn't for me, but F3... Blimey, that feeling when I first stumbled out of the vault, with an entire world of ruins stretching out in all directions... One of my best videogame memories so far. Again, many great memories from that game, but I feel I've been everywhere at least fifty times, so there's not much left to please my explorer-vibe, and it's got the same problem as Oblivion; the flaws are getting more and more glaring now.
I don't like New Vegas as much, but... It's still one great game, and I'm not done with that one just yet. Oh, and iron sights. Would it have been that difficult from the start, Beth? Would it? D:
Also, Operation Flashpoint. The proper one. Not that I wouldn't want to, but no present day computer will run it for me. :< At least I've got the ArmA-series.
(Oh, and if anyone of y'all even breathe "Operation Flashpoint 2: Dragon Derping", I'm afraid I'm going to have to hit you over the knuckles with a wooden ruler. We do not mention that monster by name.)