This is a matter of opinion, of course. I will say Jetbrains’ PyCharm. There is a free version which is really a good ide, not crippled.
If PyCharm is too complicated for your needs… i.e. you do lots of small scripts, not large projects of modules, then you might prefer PyScripter. Even that knows about projects of multiple files, but it’s much lighter and simpler than PyCharm. It has a reasonably good debugger, too.