I suggest looking through http://tesivpositive.animolious.com/, not only because it has lots of mods listed in various places, and links too, but also - and (probably) *far* more importantly - tons of excellent info and advice about mods, mod installation, configuration, tools, etc., etc.
Personally, I don't think the game is all that playable without a leveling mod (e.g., nGCD + Progress, Realistic Leveling, or perhaps Oblivion XP if preferred) and a mod to "delevel / relevel" various beings, including potential opponents (e.g., courtesy of OOO, MMM, Fran's, FCOM, maybe TIE, or any other, really).
The magic system is also irritating in several ways, as is, so something like Supreme Magicka and/or LAME, possibly with the addition of a mod like Fizzle (or similar) might be just the thing.
The UI is atrocious, being more appropriate for consoles, so either DarNUI or DarkUId DarN, or something close, is almost essential, IMO.
The wilderness gets a bit "samey", I think, so I definitely recommend grabbing as many of the Unique Landscapes (UL) as you fancy. All of them is best.
Hm... but yes, I must agree - it's very much a matter of taste. :shrug:
What do you like, and what don't you like? What seems to be missing, underdone, over the top, out of place, annoying, etc.? Are the graphics good enough? Is NPC AI? Combat? Magic? Clothing? Armour?
I suppose one thing it could really do with, especially if you've played through the whole game already, is quests. Lots of them. So... http://knol.google.com/k/toql-the-oblivion-quest-list# might be a good start, perhaps?