Thank you for all your responses. And yes, QT, AQ and AW have become my best friends!
It is obvious to me though that the best agency for me is not necessarily going to be the best agency for someone else - of course my dream agent for SF is not going to be the dream agent of someone writing Christian ficiton, so I suppose the original question needed to be framed much more clearly: Top Ten agencies in the US (based on overall sales/best-seller lists). And I really liked Eddyz Aquila's suggestion of Top Ten for Genres... Anyhow, I will head over to PW and PM and check out overall sales info.
I would like to say there are many international writers on this website and all we can do is rely on the internet for information - we don't have MFA programs, creative writing courses, university professors, or all the brilliant writer workshops/query pitch weekends/festivals you have in the States - heck, not even bookstores down the road (not one in my hood - closest one is 65kms away). All we have is the great forums at Absolute Write to cross-check agents and agencies, which is why I asked if there was a definitive list.
Thanks for all your advice.
Tea Rose.