I've completed very few (non-monotypic) genera, and those that I have completed are very species poor. Gavia (5) is my most speciose I think.
I don't keep track of how many genera I've seen or which ones I have completed, but one that I was very happy to complete is Rhegmathorina antbirds - only five species, but all fantastic! The closely related Phlegopsis bare-eyes is another one, three species but all great ones. One of my lifetime goals is to see all the species in the obligate ant-following antbirds clade (so those two genera plus Phaenostictus, Pithys, Oneillornis, Willisornis, and Gymnopithys)