Which iPhone is the easiest to downgrade?

Which iPhone is the easiest to downgrade?

Which iPhone is the easiest to downgrade to iOS 8?

After checking a few blogs, GitHub, and Reddit communities - it appears that the easiest phone to downgrade is the iPhone 5 because it does not require blobs. However, as I am completely new to this field I ask the community here.

Please do not recommend older than iPhone 4, and assume I am buying a used iPhone from a second hand electronics shop.