Which tool??????

[Dave Harris sctchbld]

It depends on what I am building. However since you did say the most used I would have to say a dremel and and one of my Good Ones (TM.) cut off disks. I say that because what I do mostly now is work on junking cars for my S scale to be junk yard and before that I was sctatching Brass steam locos and ALL cutting on both of these is done with a dremel and disk.
On buildings things change -=- there it would be an Exacto handle and an EXCEL blade . I find the Excel blade to be FAAAAAARR better than the Exacto, which breaks it point :x the second it touches any surface --even balsa!