For me it's 1/9 for figures, 1/48 for planes, 1/24-25 for cars and 25/28mm for wargaming.
But that's a bit simplistic.
For diorama building, sometimes you have to pull some tricks. To give a sense of altitude, a 1/48 Spitfire looks great above a base with 1/72-1/76 figures and buildings.
Forced perspective is the name of the game, and it's usually employed in set-building for movies (Gandalf and Bilbo were not having tea at the same end of the kitchen table in 'Fellowship'), but it's also useful for photographing your models - a half-sized background object will give the impression of double the distance between it and the foreground subject. Light it right, and use a shallow depth of field, and you'll have a great shot.
Wargaming dioramas have their own challenges, though, and even at 'heroic' 28mm scale, there isn't really a sense of difference in individual size. For instance, an 8ft Space Marine is the same tabletop size as a 6ft Imperial Guard, and although you can transport 10 loaded Marines in a Drop Pod, not a single one fits within the harnesses inside the vehicle.
This is compounded by True Scale figures - either through conversion or with 3rd-party components, you can make your marines more realistic - longer limbs and better weapons are a good start. However, these guys have NO chance to be modelled inside a troop transport, so would you even want to try?
To be fair, the compromise is logical - playing a battle on a 6x4ft mat, and using Line of Sight shooting rules, means not having scale scenery or vehicles: hills would be ridiculously large, tree coverage would make moving troops impossible, and buildings would rise through the roof of your house. For years we've pretended that we're sniping from the roof of a highrise ruined administrator's office block, except it's only 4 levels up, each at 1-2 inches apart, which equates to a 30-40ft high suburban home. Not quite as epic a concept as the game wants us to buy into.
Still, it's just another minor compromise to deal with (or ignore) - and thankfully, any water, sand and fire we have to depict will never have to move in scale. Gerry Anderson certainly had that problem with Thunderbirds and related shows. He compromised on scale, and we didn't mind at all.
Thanks for reading!