I believe that binaries such as Renat.Su mentions definitely should go into /usr/bin on debian based systems. Some other flavours use /usr/local/bin.
And, yes, as Mark says, things expected to be used by root only should be in */sbin
Its not a good idea to put binaries that users are expected to be looking for in the /opt directory, if nothing else, the default user settings don't set a PATH there and the user will not find it. And its not intended for "general purpose" binaries, more for specific ones that the user, typically a not a real person, will know the full path to.
Davo