I prefer the original Fallout games to 3. I actually like F2 more than F1, despite all the jokes and "cameos." It is probably something as simple as the cd-drive on the machine I first experienced Fallout on being a piece of trash. It stuttered and froze, so it turned the game into an almost survival horror experience for me. Will the next area stream in right?
By the time Fallout 2 hit my desktop, I'd upgraded a few times and it played without a hitch. Overall, it was a much smoother experience for me. I've probably played through it about twice as many times as the original.
I played through Fallout 3 twice, ish. I didn't go through the ending on the second run and didn't care enough to get the dlc. It didn't feel Fallout-y enough.
New Vegas was definitely a step in the right direction, but I haven't actually gotten around to finishing it yet. I don't like that as soon as I make a decision (toward the end of the game), everyone except the side I'm backing instantly hates me. My complaint is not against having an actual choice MATTER in a game. My complaint is that I do something in a bunker or a remote base or a back room where I am, functionally, alone or at the very least isolated, yet the entire game world (factions at least) react to it. I guess psychic phenomenon is pretty rampant in the Mojave wasteland.
I've had mixed experiences with the Elder Scrolls. Daggerfall was great, in theory, but largely unplayable. Everyone in Oblivion was so ugly. I wanted to buy full face helmets and just hand them out on every corner. I'm surprised nobody came up to my character, who thanks to an insane amount of time in the creation menus, actually looked like a normal person, and asked how they escaped the mutagenic calamity that befell every denizen of Cyrodiil.
Morrowind is a special case. It is one of my all time favorite games, and I have played it to death. I stuffed it full of mods, and even got into making a few of my own. I sunk more hours than I care to admit into it, multiple times. There was something perfect about enchanting an amulet with a constant dispel function, and using it as protection while you set out to kill a false God who likes to name cities after himself. Of course, every trip I took through the game dovetailed into the same end-build. Fighter-thief-rogue hybrid. I kept a ring of levitation and a cloak of water walking in my inventory at all times, and appreciated the game for allowing me the leeway to create such things, even if they had the potential to break the game.
Skyrim was fun, or at least my second trip through the game was. My first trip, I sort of took the Morrowind approach, dabbling in a little of everything, and joining a bunch of guilds to see what was more interesting for me. I'd decided to focus on the thieves' guild line as it fit my character's main approach - stealth, archery, and lockpicking. But, the end of the quest, and the decision it required went completely against the mindset I used for my character.
My second trip, I basically played as the TES version of a werewolf Conan and had a complete blast.
All in all, I'd have to say that I prefer the Fallout series. I've always had an affinity for Westerns/Spaghetti Westerns, and Fallout is easier to bend in that direction than the Elder Scrolls. Hell, parts of New Reno pretty much boiled down to the retro-futuristic sci-fi apocalyptic rendition of A Fistfull of Dollars. I can't vote against a series with that kind of DNA.