What is the ideal fitting to reduce gas line diameter from 3/4" to 3/8"?
I would run 3/4 black pipe close to the dryer location (within 4 feet).  Then assemble a Tee with fittings to make a sediment trap, a 3/4 to 1/2 inch reducer,


a cutoff valve, and then this appliance connector.



A step up in appliance connectors is this one

which automatically cuts the flow (to a tiny trickle) if there is grossly excess gas flow as when the hose is disconnected or the appliance fails in some dramatic way.

Some might omit the sediment trap.