The best ATPL to get is the one that comes with a right hand seat in a glass cockpit jet (or similar)! Converting your licence is not as easy as it seems, and will inevitably depend on your skills and knowledge as a pilot at the end of the day (ie. flight test). Converting can and is an expensive and long process, differing between countries and aviation organisations (JAA, FAA etc..). often you will have to repeat ground exams at signficant cost!
Get one that is valid and accepted in the country (or area) you intend to work in. Dont worry about conversions already!