Hmmm, let me think.
1. OOO - Absolute must, no way I could play without this.
2. Darnified UI
3. Some Monster Mod.... either MMM or WAC
4. Duke Patrick's Combat Mods
5. At least one magic overhaul. If I had to choose between SM or LAME, I would probably choose LAME, but fortunately I don't have to choose, so I use both
6. Hear no evil / no more psychic guards
7. Integration: TSL
Those are the absolute minimum requirements for me... depending on what type of character I want to play some mods might be more important though, for example as a thief or assassin I would definitely add more stealth mods and RGO to that list.