Obviously it needs to learn Rapid Spin :P
Seriously, though, it needs to be bulky enough so it doesn't die in one hit. Not being weak to Stealth Rocks is a serious bonus, as it shouldn't be weak to the hazards it is switching in to get rid of. A Poison type is nice as well, although certainly not necessary, as that means it can get rid of Toxic Spikes without even needing to spin, although of course any Poison type can do this, so it doesn't have to be a spinner. Finally the perfect spinner should be able to deal with Ghost types, as they are the main Pokemon that can block a spinner, and it works much better if the spinner can take them out instead of needing t switch to someone else.
Hope this helped!