I don't think the generalizations still exist like they used to for rental fleets. Avis/Budget share a fleet in most locations, so they're hard to differentiate anymore. Also, while Avis/Budget historically had more GM models, it's much more common to see other sedans than GM in the lots. The SUVs are more often GMs, though.
I can't speak in depth for the others as I usually rent from Avis.