The Elder Scrolls IV: Oblivion Martigen's Monster mod
Full user review
"This, quite simply is the best Oblivion mod!"
It changes the game completely, makes it more replayable, makes the game more scary and challenging and puts so many new creatures and items into the world that it's dizzying! When you add the non levelling and the creature AI you have a total winner!
Only negative is because it has so much in it, it's a 300mb+ download! Now
at 3.6 beta 2, by the way, AND JUST KEEPS GROWING!! :)