I want to know how to say, "What's your problem?" as an insult in Mandarin.
If you wish to express
What's your problem?
this in the sense of
Do you have a problem with that?
you can say
意见 means, in general, opinion, comment or suggestion. (Compare with 建议 which possesses a similar meaning yet with a softer connotation). The former may suggest there is a variance in opinions to the extent of contention, while the latter describes a suggestion in the sense of a non-dictating recommendation.
Depending on your tone of voice and context, 意见 may convey a different attitude, but it tends to sound argumentative in most cases. Consult this.
Do note that this way of expression might get you in trouble, so use your words wisely! :)