Well since your definition of "rooftop" seems to include "high floor with a view" since indoor bars are allowed, then TOTTS at The Excelsior (which has a terrace). But be quick - it's closing after 46 years on 31 March.
For a very basic bar with some snacks and a great view, the one on top of Central Pier 3 is OK.
It's a bit unfair to describe Ovolo Southside as "middle of nowhere" - Wong Chuk Hang has quite an art scene and several decent "private kitchen" restaurants around. And with the South Island Line it's only a few minutes from the traditional centres of things. The only downside (of the lower level bar at least) in my experience is that there tend to be a load of families with young kids running around when I go there early evening.
More a rooftop restaurant than a bar (the bar is on the ground floor) is Padstow in Pak Sha Wan (Hebe Haven, near Sai Kung). It's a great spot to have lunch on a nice day.
Over the years the rooftop at The Fringe Club has been available as an outdoor area for the bar sometimes, but it seems not to be at the moment.
The Ladies Recreation Club has a nice rooftop bar/event area surrounded by the mid-levels towers (but it's a members-only club).
But to be honest, the best rooftop "bars" in HK are probably those that are owned by the person with the apartment below them. Many apartment blocks in HK have some really nice rooftops which the owners have equipped with all the trimmings (I know one with a wood-fired pizza oven!). But obviously you need to know the people and get invited to those parties!