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.

Optivisor - because I suffer Presbyopia. i.e I'm getting old!