- Qualitative benefit: high-quality entities do things better than any quantity of low-quality ones can.
Quality and productivity modules.
Armour and vehicles with a grid, and most items that go in a grid.
Radar and turrets.
Bots.
Fuel (when used in a vehicle). - Large quantitative benefit: the smaller required number of high-quality items also means a smaller required number of (expensive) associated items.
Crafting machines.
Speed and efficiency modules; beacons.
Ammunition. - Small quantitative benefit: the smaller required number of high-quality items needs less space and perhaps less power, but otherwise is no better than using a larger number of lower-quality ones.
Most entities used in electricity generation (steam engines, solar panels and accumulators, etc.).
Power poles.
Roboports.
Walls. - No significant benefit: quality either provides no benefit at all, or just a health increase that is irrelevant for entities not exposed to combat.
Belts, pipes, chests and tanks.
Rails and all railway equipment.
Circuit network items.
Tiles.
Fuel (when used in a building).
For class C, you might roll the dice for quality at the final stage of construction (it's more-or-less a free bonus), but not care all that much what quality you end up with.
For class B, it might be worth developing a production line with guaranteed quality intermediates by creaming off the best at each step, but time-consuming and wasteful recycling to maximise quality is probably uneconomic.
Only for class A items might repeated recycling to get the very best possible be a plausible choice.
EDIT: Deleted bots from A class: construction and logistic bots are C class; combat bots are technically A (because of their increased range) but as consumables would not justify repeated recycling.