The most useful of all :
A range of both modelling scalpels, and box-cutter type knives.
30cm Metal ruler and a small set square.
Sharp dividers and scriber.
Razor saw and aluminium modeller's mitre box.
Small glass sheets stolen from cheap or old picture frames, used as a perfectly flat surface when glueing assemblies, or sandwiching assembies such as bogie underframes to keep them perfectly flat while drying and setting. Sounds strange but I've found glass sheet a perfectly good cutting surface too
Various good quality tweezers - pointy, dog-leg, and blade style tips.
Good quality, flat nail files - not the metal file type, more the emery board type.
Pin vise and a range of 2mm and less drills.
Kadee coupler height gauge.