As you know all major and most small publishers do not accept unsolicited manuscripts, especially from first time authors. OK so I emailed many publishers and asked them to send me details of their preferred agent(s). The invariable response was that they would not supply such information.
There are 27 agents listed in the "Australian Writer's Marketplace" alone. Overseas websites list hundreds. Please would you tell me how I can find an agent who is respected and utilised by one or more publishers - preferably big international publishers, because my novel is a very ambitious world-ranging saga.
Publishing companies won't name preferred agents for at least one good reason: in a market as small as Australia, they probably don't have any. They'll have to deal with all the agents and have probably had good and bad experiences with all of them, because that's just the nature of being in a creative business. Even if they did have a personal favourite, because they speak on behalf of the company they wouldn't really be able to say. Also, they risk upsetting the less-favoured agents and therefore not receiving submissions from them. So I guess they're in their own Catch-22 in that regard. If you asked me who the best Australian publisher is, I couldn't give you one name - each of them is good at different things, some of them are good at the same things, and all that will change over time as their personnel and business imperatives change.
In regard to how you should know which agents to query, Miss Snark would have told you to query them all. Most agents are reputable - word gets out too quickly if they're not, and even in the US it's a small industry. Just to make sure, though, there's a list of Writer Beware's 20 Worst Agents here: http://www.absolutewrite.com/forums/showthread.php?t=28961 (it's for the US). In Australia, the industry is far too small for agents to behave disreputably and get away with it for long.
In general, though, you should be doing your research - check agents' websites, see which of their authors are published and with whom. This research is part of your work as a writer - no one else can really do it for you. But Google helps a lot - Google your favourite authors and you'll probably end up at their agents' websites before too many clicks.
The main point, probably, about why there's no definitive 'good' and 'bad' list is that deciding on the 'best agent' is highly subjective, just like everything else to do with writing. Publishing is a personality-driven business in that it's the networks within the industry that are relied on more than anything, and whenever personalities are a determinant, people will always have different opinions.