Doesn't have to be the strongest but she should be very healthy, in fact it may be best to use one of two that you think are similar, this will then give you the kind of direct knowledge that no one on either side of the defoliation debate can alter. What I find exceptionally useful about a major defoliation of a bushy plant before flowering is that suddenly the entire structure of the plant is revealed which is very satisfying as you can then visualise what she is likely to do and how best to manipulate her to bend (both literally and figuratively) to your will. I don't think there is anything more spectacular than stripping a healthy plant almost bare and wondering 'what the f have I done' and then three days later it doesn't look like you've done anything.
So you defoliate to have a look at the structure, or to increase airflow or just to get light all through the plant. I used to do more severe defoliation at very early stages, and while I got good results, over time I have abandoned this as not really necessary. Normally I'd suggest to go easy because I know that it feels kinda wrong to do it at first but that is just our mawkishness showing. Some phenotypes are naturally less leafy and don't require any leaves off at all for light to get through.