Just yesterday, in response to a challenge, I managed to narrow down a list of my top 5 mods which are not the UOP, Streamline or a world scaling overhaul. Three of them were my own, and of those, two might not be best for your first time around the modblock. So, I'll clip those out and just give you my top three.
1. http://www.tesnexus.com/downloads/file.php?id=2037When you pick a flower, harvest a tomato, pull the cap off a mushroom...
you see it in-game. You can know, just by looking, which plants you've picked and which you haven't, and later, you'll know when they grow back.
2. http://www.tesnexus.com/downloads/file.php?id=14065This one's mine. The project began planning before Oblivion was released, and I only really became satisfied with it within the past year. It completely replaces the level advancement system with one that scales smoothly: your attributes rise automatically along with your skills, and while you gain levels, there's no special event associated.
Advancement overhauls are one of the areas where it's widely accepted that something has to be done, but there are several options which different people prefer; in the interest of fairness I should point out two popular and actively-supported alternatives: http://www.tesnexus.com/downloads/file.php?id=13879 (which is fairly similar to nGCD), and http://www.tesnexus.com/downloads/file.php?id=15619 (which is very, very different).
3. http://www.tesnexus.com/downloads/file.php?id=22819Reduces or removes ambient lighting in caves, dungeons, basemants and other places. The point is: torches aren't worthless, and Light spells aren't a waste of Magicka! While it seems like a fairly simple change, in my opinion this mod does more than any other graphical enhancement to improve the environment, immerse you in the game, and instill a sense of danger and adventure.