Falling out of the Wild Mass Guessing about the new companion on RPGnet, Gideon proposed this classification system for companions rating them by Tech and Feisty.
I'd say general levels of Feisty have increased over the years - Ian and Barbara were more Feisty than many later companions, but they were paired with a low-Feisty Doctor. The number of companions may also be a factor as there's only so much Tech and Feisty to go around, as is the Feisty level of the Doctor.
Scott Gray writing the Eighth Doctor comics suggested a binary system of Girl Next Door and Mrs Peel - ordinary people versus warriors, spies, aliens and superhumans.
Of course there are plenty of fringe cases - Sarah Jane varies in Feisty level or GND/Peel-ness from adventure to adventure and becomes totally Mrs Peel when she returns in 2006 - and characters who change, generally rising in Feisty with experience and occasionally becoming godlike entities.
Where does your companion fall?