My vote goes to IB.
It is, IMHO, more rigorous than STPM and also have a very excellent reputation worldwide. By far, I believe, the most recognised Pre-U in the world, even more than A-level.
Try to look up Aus, UK, Can, US's universities websites. I bet you can hardly find a university that doesn't list out the entry requirement for IB.

I accidentally clicked null vote and in the end failed to make a vote.

STPM, for its rigorous and wide scope, prepares students very well, too.
A-Level, which is quite similar to them, should be okay as well.

As a SAM graduate, I will say it doesn't prepare all-rounded students but prepares best prepared students. Students are able to cope with assignments all that as the workloads are expected to be lesser than SAM time. Nonetheless, it's noted that Specialist Mathematics should come as a must for Engineering as it simplifies your life a lot in university. Meanwhile, I believe the covered scopes should be sufficient for university entry.
I believe AUSMAT should come very similar with SAM, be it from WA or VIC state.

I have bad impressions on ICPU that I find that graduate (ONE sample only) is unable and unmotivated to cope with studies. Also, I believe they do study more subjects but less in depths (America system) which makes them harder to cope in a UK system (or in Aus system) which stresses more on depth than breath. (But, I see Aus is trying to look for a better balance in depth/breath. For instance, the Uni Melb Model 3+2 Master, which will be followed by UWA in few years time.)
If you asked the opposite question, then my vote goes to ICPU.

Well, just my 2 cents.