I'll have to make some assumptions - You're in Ethiopia and your company won't let you take an ATPL test until you upgrade so you want to get an ICAO ATPL by yourself? The problem you might have is lack of PIC time, often MPL holders have to hour build even to get to CPL level and some authorities don't even have MPLs which complicates things.
UK or EASA are pretty straightforward - Medical, Exams, Sim check but not exactly easy because you have to pass 13 exams which takes 6-12 months.
FAA has much easier theory (1 exam) but you have to do a mandatory ATP-CTP course (kind of like an MCC) then a type rating course so it's much more expensive. There is the option to take the test on a piston twin with all the extra training that involves but I'd stick to jets.
If you have 150 hours of actual PIC I would probably go for a Canadian TCCA ATPL it's kind of between the two - you need 3 exams and a sim check. If you've got money then FAA will be the quickest. If you've got time on your hands then UK and/or EASA will be the cheapest and give you the most options.