Great question, reader Pip, if that IS your real name. Silly of me not to post about it before. In general you want to use the same sorts of apples you use for other baking baking applications: firm, crispy and flavorful. I think red apples are best for candying: gala, fuji, cameo, pink lady, Jonathans. For caramel apples, the Granny Smith is the ultimate in my book, but Golden Delicious and Jonagold work well if tartness isn’t your thing. Really any of the red candy apple varieties work too.
I also suggest going for small apples when you can find them. Usually these are the ones sold in large bags in grocery stores. Really big apples look impressive when they’re coated, but smaller ones gives you a better ratio of apple to topping.
As for varieties to avoid, stay away from red delicious and Rome apples for sure. I’m personally not a fan of McIntosh or Braeburn for this purpose. Thanks for the question, Pip!