Good question rooky2. I hate to avoid giving you a direct answer but it depends upon your budget, what you are looking to achieve and over what timeframe and to some degree, your risk profile. And even when you have the answers to these questions, it often comes down to the quality of the individual properties rather than just one specific suburb. So for example you may have been focusing on say, Newtown but then you find a cracker in Alexandria.
Not long ago, the Sydney Morning Herald and Domain had an article on the best investment areas in Sydney. Lakemba was #1 for return on investment $. Naturally this would also include some of the surrounding suburbs with similar prices & rent return.
At Professionals Belmore we do have properties in Lakemba, Belmore and surrounding areas for rent, and offer a full property management service.
Even once you find the "best suburb for investment" a lot will depend on the type of property you buy and if/what you are prepared to do to improve it. While rents have been increasing quite a few potential and even existing tenants are wanting better value for their money. For instance a unit with a lock up garage will rent faster for higher rent than one with only a carspace or street parking. The same can be said for mirrored wardrobes and more modern kitchens & bathrooms. The better condition a property is and if it has had updated features & fittings, the better rent, shorter vacancy time & better quality tenant it will attract. That will be the same no matter what suburb you buy in.