Well, I'm trying to export, for exmple, General Ledger Report in CSV or XLS format. I was searching some ways to do that in Odoo 9, but I don't know which is the best way.

Some advice?

To my knowledge, From Version 8, Odoo has stopped adapting the Jasper or Ireports reporting mechanism.

Odoo 8 and 9, strongly follows the Qweb Report, which is the nothing but a template report engine readily available to build all the user-client/pdf accessible reports such as Invoice Report, Quotation, RFQ etc, as well as for generating statistical analysis reports under Reports and/or Dashboard menus.

However if you still wish to develop your custom reports using third party reporting tools, then you can go for Pentaho reports, which can be easily integrated into Odoo.