Very hard to answer. It depends on your nationality, academic accreditations/professional background, and financial situation.
UK, Singapore, EU, Canada, New Zealand, Costa Rica, and Panama can be relatively easy to get into if you can demonstrate that you will bring value to your new host country. It's even easier with a local employer sponsoring your visa, but you can do it on your own if you bring somewhere in the range of 100 to 500,000 EUR/USD equivalent in personal wealth, real estate investment, or provable business income (bonus points if you hire locals).
But if you are just some average person with average means and no local employment sponsorship, there are no ways in (besides illegal immigration, asylum, or refugee status).
Do not know what you mean exactly by developed countries, but I suggest Argentina as one of the easiest countries to immigrate and get a passport in 2-3 years.
There are more than 10 options of immigration there.