Well I figure I could get us started on this here for our companions. Certain gift items do more then others on certain companions.

This is what I have so far so if you see something I am missing go right on and post it.

Jorgan: Ammo Belt, Survival Gear, Padded Weapon Case, Military Gear
Mainly any military items seem to do the deed here.

Yunn: Tech, Cultural Art, Memorabilia

Elara: History Holo, Hutt Data Lib, Banners and luxury goods

That is all I have for now so don't forget to help me out here.

This has gift info for each companion: http://www.swtor-spy.com/companions/

C2-N2 isn't listed, but I don't think any gifts work. I'll try to check tonight.