Whats the best suburb south of the Adelaide CBD?

I don't think there is a definitive answer to that question!
We rented in Westbourne Park a few years ago and loved living in a garden suburb that was within easy walking distance of the Mitcham and King William Rd shops. The public transport was excellent there, too. One set of neighbours was unpleasant, but you get that anywhere and the other neighbours were friendly.
South Brighton is beautiful, but a bit further out. All of the beaches I have seen south of the CBD range from nice to stunning. Marion has the new swim centre, good transport and facilities. St Mary's is very green and still has a lot of big blocks of land. Overall, there is a lot going for the suburbs south of Adelaide, and most people would find something to suit their personality type. The one thing to take into consideration is commuting time. I know some people who live in Seaford Meadows and work in the CBD, and it takes them an hour and a half to get to work, and the same amount of time to get home again.