I assume you are looking for cityscapes?
For HK Island I would suggest the peak - the area around the peak tram is good but very busy. From the peak tower, walk around Lugard road for some great views over Western district and the harbour. Off lugard road, there is also High West (a short spur) which gives great views (depending on visibility) over outlying islands.
You could also walk along Bowen Road for views over Wanchai.
For Kowloon, I would suggest Lion Rock (you will need to hike to get here) which has great views over Kowloon. Alternatively, Fei Ngo Shan (Kowloon Peak) - you would need to walk or get a taxi here - Fei Ngo Shan isn't as close to the city as Lion Rock.