If you are just getting started in Singapore, the best value prepaid seems to be Starhub. BEWARE: You get charged for incoming calls in Singapore with StarHub.
Honestly Starhub's 3G doesn't work at times. Worse than Berlin. I've used their service on and off for year, with different devices. Problems don't get fixed. It's absolutely crazy.
So I've been investigating (their Websites are shit, so I've actually queued up at their shops in Bugis) the options and they are not good:
Both their entry level contract plans are ... 39SGD pcm. 2GB is included, 4G is included if your phone is capable and ~5SGD for extra gig in excess.
M1 is seemingly better because if you take two lines out with them, you can get free calls between the two. Also minimum contract with M1 is 3 months, instead of 24 months with Singtel.
You can only open a contract if you physically have the full employment pass, not a preliminary one.
Update: Switching to StarHub
You can check if you can Fibre optic here: http://www.opennet.com.sg/
If you can't get Fibre Optic, you are usually stuck with 10Mbps ADSL for 49.90SGD a month.
It's worth noting that even the allegedly fastest Fibre in Singapore is probably not going to get much better than 100 KB/s to Europe.
There is no online billing until you get your first paper bill and then you need to manually switch over.
They won't warn you if you go over the 2GB quota. In order to avoid getting charged for going over quote you must go to Settings > General > Usage > Cellular usage and remember to reset statistics monthly.
There is a 26.70SGD charge for the SIM itself. Plus a 10.70SGD registration charge. They don't tell you this until you sign up. haha ha...
They cannot port a prepaid number.
Do NOT roam on this, is the advice of sales representative in Bugis. Even in Malaysia, it's too expensive.
Unlike Malaysia, you do have 'a lemon law' protecting your consumer rights for products, but not services. You can complain to CASE though. There is no cooling off period to evaluate if the network works for you.
The good news is M1's 4G is quite nice when it works. Now I can skype, listen to BBC radio and watch Youtube videos on the go. However there are some issues. Like you can't upload on the IOS6 YouTube in HD which sucks. If you think 4G is some how seperate and better than 3G, it's not. LTE 4G fell over during New Year's Eve celebrations.
Online customer service is non-existent.