I've just made a whole load of these for my own personal project !
I did it with simple planes, bend deformers, mograph, aerodynamics, and wind forces.
The stages involved are fairly simple, being something similar to the following:
1. Get leaf photos - I literally went outside with a camera and collected 8 'reference' Autumn leaves in various colours and shapes of the season. Maples look most attractive on the whole. Or if you can't be bothered to do that you can find some online.
2. Scan or photo the leaves (on a white background), separate into individual images, then remove the white BG in Photoshop etc and save out as png or any format that supports Alpha.
3. Create a plane for each leaf type in Cinema, and give it 1 segment down the spine, and perhaps 4 or 5 down its length. You don't need any thickness. Then each one of those gets a bend deformer (fix Y length) which allows you quickly set the degree of leaf curl per model.
4. You can load your image into the colour and Alpha channels of the material to get very low poly yet realistic leaves.
5. Next stage is to place those leaves under a cloner, but the mode you choose there will be defined by the initial arrangement of leaves you want in your scene. I started with a Grid Array of them off-camera, and used dynamics, gravity and wind to make them fall and flutter down past the camera. Doesn't have to be a cloner though. An emitter might work better for you if you need to generate a constant stream of them for example...
6. My Rigid Body (leaves pretty much ARE rigid bodies in this context) tag went on the Parent Cloner, but was set to apply to children / Top Level etc so it affects the leaves individually. Then it's just a question of making sure Aerodynamics and 2 Sided are turned on in the Forces section of that tag, and giving it some suitable Lift and Drag (I found you want roughly half as much drag as the Lift value). I then Added a Particle Wind Force, and added it to the list in the Rigid Body tag's Forces Tab, making sure that is set to Include and that the force type is Aerodynamic Wind.
7. Then you just need to experiment with different wind / turbulence parameters to get the type of movement you want. Everything else in your scene that the leaves might have to interact with will need a Collider Tag. I suggest you set friction very high, and bounce very low both on the leaves and the colliders. Also make sure your Dynamic Scale is set to something appropriate relating to the LEAF SIZE in Project settings / Dynamics / Advanced. No good will come of having leaves that are 3 cm long, and then having the default of 100 cm there. I set mine to 10 cm (average length of my leaf objects), which gave much better results.
If you want consistently whirling leaves that don't settle much, then I'd also make a large 'container cube' with a collider tag on it so none of the leaves escape the scene area and continue to get blown about without getting blown away ! Put a display tag on that in Lines Mode so you can see what's going on inside it...