I would go with Munns real estate in Carrum Downs, I purchased a place through them 6 months ago and found them fantastic to deal with and made the whole process painless. All round I was very impressed!!
It depends on what suburbs, because a lot of suburbs are dominated by different agencies, but I would probably start with Hocking Stuart because I love their very well designed books they release. I read them on the weekends when having breakfast, and am always impressed at how professional yet fun they are. BUT I would then ask friends next who they have gone with.