I use the following;
Studio Builder Suite - I use this solely for moving grouped objects along a vertical or horizontal axis, instead of the strange orientations studio may sometimes give.
GeomTools, Gapfill + Reflect - I typically use Gapfill when dealing with extremely small parts, such as part of a wall's corner. I also use it when dealing with holes that regular parts may be too tricky to use. I use Reflect when dealing with symmetrical builds and doors, and I find it to be a very handy build when copying the foundation of a build over.
Archimedes - I use good old Archimedes for curves, and I find it to be especially helpful in tunnel designs such as the industrial themed or Stargate themed builds I've made.
RopeMaster - I use this mainly to build curved pipes or wires.
ToggleHandles - I use this when building on a macro level and transitioning it to a micro level. An example of this can be found on my scientific hallway designs, with the grey plates having been macro built and then downscaled with the assistance of this plugin.
3DText - One of my favourite plugins that's able to make excellent text designs, I use this when 2d text may be inappropriate or insufficient for the desired purpose.
Camera Light - I use this when dealing with darkened builds where I haven't built lights yet. I find it especially helpful for building doors.