What's the best Oracle E-Business Suite module? Depending on the salary and the opportunity to find a job.

Oracle ERP have traditionally done well in Financial modules, followed by HR, CRM , SCM.
But it is also widely used in HR modules in the Middle East market.
GRC, Transportation, ASCP are also few of the modules that are gaining traction.
If considering salary and opportunity I'd rank as Financials (AR, AP, GL, CM, FA) > HR (Core, Self Service, OTL, Benefits) > CRM > SCM (Manufacturing, OM, WIP, BOM) > GRC, Transportation etc.

I would recommend to combine either HR and Finance as first preference or to go for Finance and SCM, if to chose HCM only it is very recommended to be techno-functional consultant, but combining two core areas will stand you out of the crowd.

Oracle E-Buziness suit has many group of module, one can take it as profession by learning and gaining any group of the modules, Like Financials, Supply Chain, Manufacturing, HRMS, Projects, Transport Management etc..
Salary would be depends on years of experience and kind of knowledge if it is Implementation or Support or Upgrade.