My favourites for achieving this from RTR OO based on numbers purchased:
Bachmann 16T minerals,
Bachmann 12T general merchandise vans,
Bachmann high steel 13T general merchandise open,
Airfix GMR / Hornby , five plank 12T general merchandise open
Airfix GMR 12T sliding door general merchandise van.
Fine efforts such as Bachmann's 13T wooden minerals, Oxford Rail's LNER type six plank 12T general merchandise open, Rapido's LMS type five plank 12T general merchandise open, haven't been purchased in the quantity that might be expected: because I had already built a great many kits of these types in the past.
Let's hear it for the ordinary wagons that went everywhere, and carried practically everything when the railway still moved most of the nation's freight! Once you have a hundred of these types you can allow yourself 'exotica' , but only one per hundred, from a selection such as: tank wagon, cattle wagon, conflat or lowmac; and don't even think of a bogie bolster, grain hopper, blue spot fish van, just yet...

