Hello there,
I shall try to answer this question by pointing out the pros and cons of each subject as far as cracking the IAS examination is concerned:

Humanities

Humanities have traditionally been termed as the bastion of civil services examination. If we look at data released by the UPSC in their annual reports, we can see that humanities subjects have had a higher success ratio compared to others. Despite, this trend the general participation and success ratio of students belonging to humanities background has seen a downward turn in the last few years. But, despite all these factors, candidates with humanities background still forms a considerable number of aspirants cracking the IAS examination.

Engineering

Under the current circumstances, we can clearly see that engineering has been the mood and way as far as cracking the IAS examination is concerned. If we take IAS toppers from last few years, it is clearly evident that candidates with engineering background have dominated the examination. While the success ratio of candidates who pick engineering subjects as optional in IAS examination is still on a lower side, we can clearly see that toppers have succeeded with technical subjects. But, in general, engineering is surely the go-to stream, if you want to not only crack the civil services examination but also emerge among the top ranking candidates to be eligible for the IAS posts.

Science

A small but respectable number of candidates from graduates from science stream do take up civil services and crack it regularly. However, their number has not really been great enough to leave an impact on the final tally. But, considering the growing clout of scientific thinking and rising need of technocrats in the bureaucratic setup, it is surely seeing a slow and steady rise.

Medical Science

Medical science is another similar stream that has often been written off as far as IAS Examination is concerned. This is primarily due to after attaining a professional degree, not many candidates are looking to join the bureaucracy. But just like physical science subjects, the number of aspirants and successful candidates with a medical science background is on the rise.

B.Com / MBA / BBA / CA / CS / CFA

All these degrees are again considered to be professional in nature and therefore draw a negligible number of candidates in the overall scheme of things. But in last few year, we have surely noticed a hike in the candidates from B.Com and MBA background have seen. With the introduction to CSAT which focuses on reasoning, mathematics, data interpretation and decision-making; candidates from B.Com and MBA have started thinking more seriously about the civil services examination. In coming days, we should expect to see a significant growth in their number in both participation as well as success ratio.

Conclusion

Irrespective of what the past and the current trends suggest, as far as cracking the IAS exam is concerned, it all comes down to the passion and interest of the candidate. Even if you pick the so-called ideal steam or the subject, it will not matter until and unless you really are passionate about it. Along with passion, the stream that you choose should also inspire you to study well and study hard as there is no replacement for hard work when it comes to the civil services examination.

Hope this helps