Doing b. Tech is best or degree is best for upse??
From the question it seems you are equally interested/disinterested in the three options (BA, B Sc, and B Tech). In such a case, I strongly suggest a B Tech, as it provides the maximum options in case a) you change your mind about trying for the civil services (it is known to happen; some people discover new career opportunities they had never had exposure to before, or get disgusted with repeated tales of corruption or honest officers being punished for their honesty), b) you attempt the exam one or more times and don't get through, or c) you get through but later want to leave the service. Also, if you intend to opt for higher studies in the west at some point, most universities require you to have four years of prior college education before you become eligible to apply for their degrees (e.g. MS, MBA etc).
Having said that, if you have a strong interest in a particular field (be it BA or B Sc), you can opt for that. The UPSC does not discriminate based on the nomenclature of your degree, I know people with all sorts of graduate or postgraduate degrees who joined various services. However they would naturally be more impressed with someone from a respected, hard-to-get-into institution. So try to get into the best possible institution in your chosen field.
UPSC study is mainly about awareness of events in India and the World, and its history, geography, economy etc. As of now there is a specialised optional subject (may or may not remain). If the optional is the same as your graduation subject, life is easier (only if you are passionate about the subject, not if you are doing it as a formality). But it's not something one should stake her career on. If you do a BA in Sociology (say) just because you want to take Sociology as an optional, you would be left stranded if a) Sociology stops getting marks suddenly in the UPSC exam — all subjects pass through good and bad phases based on UPSC's whims and perceived oversupply, b) the system of optionals is stopped altogether, or c) you are unable to clear the exam. Plenty of engineers and doctors get through each year with humanities subjects. It's perfectly doable with the right materials and planning.
Bottomline: choose your graduation discipline based on your interest and the field's inherent career prospects, not dedicatedly for UPSC alone. UPSC is passing through a flux, there have been many changes in recent years to exam patterns that had stood since decades. More changes may yet come. So minimise risk and maximise potential opportunity for now.
