What is your favourite Mod?

hmm, I see. I usually have stuttering when changing cells, I guess the only fix for that would be a mix of ini tweak for surrounding cells, really much RAM and multiple processor cores with at least 3GHz. One problem of Oblivion is that in some cases, it can overload itself which causes slowdowns. Those usually appear after 1 or 2 hours of gameplay. Streamline actually fixes that, but it seems that that does not work for everyone.
My PC barely runs Crysis on mid-high...

Having modded Oblivion it was quite surprising to see how many active scripts were running.

I think most of them run multiple times every second. It seems such a waste of resources to me. Some modders make things worse by creating Quest scripts which constantly execute more than they need to.

The following list applies to Oblivion,

1.) Kvatch Aftermath by Giskard - It's such a cool mod, and so perfect to run once you finish the mainquest in Oblivion. I run v6.8 but as far as I can tell Giskard pulled it entirely. It's not easy to find anymore.

2.) MRI soon to be MAO - Once mixed with loads of custom animations Mayus mod really shines. It makes my character feel so unique! Congratulations Mayu your mod has become my new second place. Well so long as it's supplemented with loads of custom animations like DMC Stylish Combat, Spellsinger, loads of Seph's mods from hand to hand, to the run animation from his dual wield beta, from the kick up animation. There's also the cartwheels from Seph's acrobatics almost forgot those, but I don't like the dodge back, and forward animation in Seph's acrobatics. Too much flipping for a simple dodge. The vanilla back flip and roll forward suffice. Also Amazing Jump is my favorite jumping animation. Nevertheless, thanks to MRI, and MAO only my character utilizes all these animations, which makes me feel "special." Thanks again Mayu.

3.) Blood and Mud - It feels like an official expansion. I mean even if it had no quests whatsoever, the overhaul of Bravil always blows my mind. I can't wait for the MAO patch for this one.

4.) Beautiful People - I've played quite a few custom races in Oblivion, never much caring for the Vanilla races as they are just too ugly for my tastes. There are a lot of cool races out there, but I always wind up going back to Mystic Elves. I guess they're just my race of choice. I prefer using them in Beautiful people because you get a much larger selection of eyes to pick from, including glowy ones.

5.) OOO - I no longer run OOO sadly, but if the MAO patch for it goes through I'm going to try to run it again, even though I'm in the middle of a game. But I do believe you can apply OOO to a saved game via Wrye Bash if I recall? It would be great to run OOO again, I really do miss it. Almost as much as I miss Blood and Mud. "Almost."

Honorable Mention.) Longer and Intense Defense of Bruma - It's a very, very small mod only changing one quest. But it really makes Defense of Bruma worthwhile by making the fight last for 10-20 minutes. I prefer 20 minutes, it really makes the quest feel epic. Grant you, in order to make it through this one you'll need to lower your video settings as loads, and loads of enemies wind up flooding the screen killing your FPS. You'll need to get as many soldiers as you possibly can. You really can't be lazy about gathering troops prior or you will fail this quest. You need troops from every city. Also make sure your restoration skill is 100, and make sure you have Superior Convalescence or Martin will likely die.

Wow that's my top 5 off the top of my head, with that honorable mention. There's still a ton of mods I'm forgetting that are must haves most likely. Also there are others that aren't necessarily must haves, but still great mods. Too many to list off, so we'll stick with those 5 and the honorable mention for now.

I really like Fakefactory's series of Cinematic Mods for Half life 2. It renewed the whole HL2 series for me ever since I first played it last spring. The game looks so much more real with it that I was shocked the first time I ran it!

For Oblivion, easy: Bobos Summoned Chest