it is hard, for the exact reason you've given - there are lots of classic directors where i've cherry-picked their best, but haven't dived into the rest, so i can't really give a proper judgement. i've seen lots of films i really like by kiarostami or carl theodor dreyer, but it doesn't mean much to say xxx is my least favourite. sean baker's last three films are three of my favourite films of the century for sure, but i haven't his other five films, so i don't think it makes sense to say that tangerine is my least favourite when i have no idea what the majority of his other films are like.
i wouldn't necessarily say these are my top five directors, though they're all definitely in the mix, but they are definitely the five i like the most where i've seen enough of their films to say firmly 'that one is my least favourite' and have that actually mean something.
guy maddin - keyhole
yorgos lanthimos - alps
whit stillman - damsels in distress
russ meyer - beneath the valey of the ultra-vixens
park chan wook - stoker