Anything related to my studies, if it's asked by people that aren't studying natural sciences, math, medicine or informatics.
You see, I'm studying molecular biology, which is a field I find really fascinating. It's not the most obvious job-grab in the world, but there is a market for it within medical firms, disease detection, brewing, foodmaking, research, etc. Whenever I tell most people what I'm studying, one of the following generally happens:
1) "Huh. That sounds.. interesting." and then changing the subject. Always said in a semi-condescending "wtf am i listening to"-tone.
2) "Huh. That sounds.. interesting." followed by awkward silence because they have no idea how to reply. Gawd. Economy students and law students are the biggest offenders when it comes to this. Gah.
3) Other people in the conversation starts complaining about how much they sucked at math/biology/chemistry/physics in high school. Which I really don't give a shit about.
4) "What are you gonna do when you're done studying?"
I definitely prefer the last one. My standard reply to that is "mad scientist that will eradicate humanity with ". Which has gotten me both promises of support for my future research, some good laughs and women replying flirtingly that "they should get on my good side so that I let them live, in that case".