Which question is most important to consider when deciding upon a contraceptive method

The most important question to consider when deciding upon a contraceptive method should be: why do you want to use a contraceptive method?
From this question answer, you can divide the contraceptive option.
If you want to prevent pregnancy, then how long? Short term contraceptive method might be more troublesome than the long term one. Condom might be better for those who aren't married yet.
If you don't want to pregnant again, a permanent one is a better option.

You should also exclude medical disease that might be aggravated by the contraceptive, but one question won't be enough to do that.