See http://forums.silverstackers.com/message-181902.html#p181902 - the Mouse and Ox have sold out, but the Tiger and Rabbit have not. Also see http://forums.silverstackers.com/message-181326.html#p181326 - the linked presentation has the Series I mintages, most were in the low hundreds. Niche collectible but they have a strong following.
Interesting fact about the 2000 1kg dragon w/gemstone. Mintage of 5,000 however only 500 were reserved by the Perth Mint for sale to Australians.
If you want to know which have had the most sales (one definition of 'popular'), check the mintage of each year here: http://www.perthmint.com.au/numismatic-mintages.aspx
At spot price they would be okay as they are considered bullion? Dragon series 1 would be the most popular like the 1oz silver coin?
You could always use the APMEX barometer, which is of questionable reliability .... Series 1: http://www.apmex.com/Search/Default.aspx?Search=diamond eye Highest premium is Dragon, then Rabbit/Snake/Horse, then Mouse, then Goat/Ox, then Rooster/Pig Series 2: http://www.apmex.com/Search/Default.aspx?Search=gemstone eye Similar premium for Mouse/Ox/Tiger (yes Tiger is sold out on APMEX, but still available from Perth Mint)