The most popular diecast scale is 1/18, offering a good balance of detail and size for collectors. Other popular scales include 1/43 and 1/64, each with its own advantages. 1/18 scale models are known for their high level of detail and realistic features, while 1/43 scale models offer a wider range of cars due to their smaller size and lower production costs. 1/64 scale, often associated with Hot Wheels, is popular for its compact size and affordability, making it ideal for large collections. 
Here's a more detailed breakdown:

1/18:
This scale is favored for its detailed features, allowing for a high level of realism and working parts. However, it's also the most expensive and generally limited to popular car models. 

1/43:
This scale offers a wider variety of models due to lower production costs and is a popular choice for collectors with limited space. Many 1/43 models are sealed and may not have opening parts. 

1/64:
This scale, often associated with Hot Wheels and Matchbox, is known for its affordability, compact size, and vast selection of models. It's a great starting point for new collectors. 

1/12:
This scale is known for its large size and exceptional detail, but it's also the most expensive and has a limited range of models available. 

1/24:
This scale offers a balance between size and detail, with many models having detailed engines and other features.