In order -
from MegaTraveller - special duty roll during character generation, the Imperial Encyclopedia is always on the table for equipment.
from TNE - I still use FF&S to build stuff and describe technology
from T4 - armour as damage reduction, psionics, CSC, and Emperor's Arsenal are always on the table for equipment and gun ⌧
from T5 - all the makers, the technology (it's easier to say which bits of T5 I don't use - the task system and character generation)
from Mongoose Traveller - boon/bane, CSC, Vehicles, Robots