港島區國際學校

GSIS, HKIS are academically strong. For SIS, it has a well-balanced English & Chinese(Mandarin) program. As I know, Kellett School has more foreigners. CDNIS was an famous IS on HK island.
I like SIS more cuz it has a stronger Chinese program comparing with other IS, kindergaten section students can also speak fluent English & Mandarin. But most of their students are Chinese or Asian, the percentage of foreigners are a little bit lower than other IS.
If you want a better English environment, you can consider GSIS or HKIS, although their Chinese standard are lower than SIS, they are academically strong, students from both schools behave well too.
The ISF Academy could be another choice for you, it's located in Cyberport and is a bilingual school, it's also running IB program for secondary years. In primary years, the school emphasizes Chinese(Mandarin) very much, their Chinese standard are very high, they use Traditional Chinese instead of Simplified Chinese. Their English standard are good too but to be honest the English standard of GSIS & HKIS are much more higher.
Sorry I don't really familiar with The International Montessori School so maybe I can't help you with it.
Hope I can help you:)