US is easy or difficult, depends on the lawyer you use, and the immigration category you can qualify for. Example, if you are an outstanding researcher or performing artist with lots of things to show (awards, albums concerts, research papers, etc.), then you go to front of queue as EB1. Greencard in 1 to 2 years, no ties to any company. Anything less means EB2, which implies labor department certification, sponsorship from a US company, lots of waiting, cannot leave sponsoring company for a period of time, etc.
For just working in the US, easy for Singaporeans. There is a special work visa category reserved for Chile and Singapore due to FTA. The visa is a H1B1 (normal one is H1B). Annual reserve/cap for Singapore is 5400 per year. Last year, only 800-900 used. You have to find a company that wants you and is willing to partition for you. H1B1 is not meant for immigration, so interview focuses on your desire to return back to Singapore. You must show intent to return. Once working in the US, you can apply for I-129 and go for immigrant visa. I'm not clear about the details, you need to talk to a good immigration lawyer.
The immigration lawyer makes or breaks your application. Good ones have long standing relationships with immigration officers, and their applications are always viewed more favorably and are processed easily. Get an inexperienced or unconnected lawyer, and you can forget about success.