I'd like to ask what magento version is it better to develop a module for so that it would be compatible with most versions.
Any professional opinion?
This really depends on what your plugin does.
I'm sure that there are plugins for 1.0 that work on 1.9.
But you have the highest chance starting at 1.7.
I say this from experience.
All my extensions support the 1.7 version and most of them work out of the box or with minor chances on higher versions.