What i use:
Oblivion Script Optimization - Makes a lot of improvements on the scripts used along the game, making them be more efficient. Must have.
Unofficial patches (
Vanilla game,
Shivering Islands,
DLCs) - Fixes a lot of what Bethesda didn't bother to fix. Extremely recommended.
DarNified UI - Changes the UI to look like one made for a PC game. Get it. NOW.
Natural Environments - Adds dynamic weather, ambient sounds, insects and other stuff that make the world feel a lot better. Get the
OBMM script as well.
Better Cities - The mod that makes Bethesda's city designer hide in shame. Just look at Skingrad and the Market District of the Imperial City and you will know why.
Illumination Within Revived - Makes the windows of the cities light up at evening and off at dawn. Good for realism.
Unique Landscapes - Several city exteriors and other landscapes replaced with a much better version, same way as Better Cities does with the cities.
Mart's Monster Mod - Adds a crap load of new monsters, tame wild animals (Foxes and bunnies!), a new crafting system with new ingredients dropped from the monsters, etc.
Alternative Start by Ship - Start the game in Either the Imperial City or Anvil docks and skip the whole prison dungeon prologue.
Reneer's Guard Overhaul - Changes the way Guard react to your crimes, making it much better.
It seems that if you use this one, you should not use the Weather part of Natural Environments. They are incompatible for some bizarre reason.
Jounks Polearms - Adds polearms to the game.
Dwemer Technology Part II: Dwemer Skyship - Adds an Airship! You can only use it as a portable house though. No flying unless you use a mod, which isn't very stable yet.
Furblivion: I made this XD. Replaces the human races (and dark elves) with cat and fox races :P
Huge quest and new lands mods, like
Elsweyr (Kittie's land!),
The Lost Spire.
Now, regarding Mart's Monster Mod, there are other two big know mods like
Francesco's Leveled Items and Creatures and
Oscuro's Oblivion Overhaul which can not normally coexist.
For them to coexist you need
FCOM, which makes the necessary changes to them so they can all run together relatively fine (Follow the instructions, else all will go to hell).
Given that i had some problems making it to work on my game, i decided on using only one of them, Mart's being the lucky winner.
Oscuro's Oblivion Overhaul should be your choice if you want to have a really big change in Gameplay. Check the wiki if you want to know what it changes.