IMO, majors ranked according to how tough it is to get in would be:
1. Banking and Finance - This is SUPER popular. A majority of the students in NBS want to get into this major, so you have to do pretty well for your Year 1 finance module (this year the cut-off was B for that mod... but even then I dunno if you're guaranteed the major) and probably do decently overall too, to qualify.
2. Tourism and Hospitality - Also pretty popular, though the admissions process is relatively more "lecheh" than for other majors. You need to submit a statement of intent (explain why you wanna take THM), attend an interview (I never attended it so I dunno what it's like :/), and get at least B for your marketing module. Basically, to get in, you have to speak and write confidently.
3. Actuarial Science - limited spaces for this one, but also little interest among the general NBS population, so it's actually easier to get in. Basically, you have to do decently in your year 1 stats mod and other finance/math-related mods like Accounting, Financial Management etc. I put this as my first choice for streaming despite my less-than-stellar grades (no kiddin') and I got in, so... yeah. Though whether or not I can survive in act. sci. is another thing! Haha.
4. Marketing? - I have the impression this is one of the more popular non-Banking and Finance majors.
5. HR/IT - least popular. Guess they're deemed less glamorous? :S
More about the streaming process...
1. You go to an online platform and rank the majors according your preferences.
2. You get the results.
3. You have the option of appealing for a change within about a week or less.
For more info on streaming criteria check out: http://www.nbs.ntu.edu.sg/undergrad/..._Streaming.asp
However, this is only a rough guide. Enrollment still depends on the popularity of the major. For less popular majors, you may not have to do as well as is stipulated. For more popular courses you may have to do better than is stipulated. >_>