Which is best board for all application design and development using IoT.
A good all around processor is the Pi 4 because it is well support and very good value for the money. But any given details can change that answer of course...
"Best" has a lot of wiggle room in it.
Best price would be ESP-01. It's hard to program and requires special 3.3v power input.
Best price for an easy-to-program and easy-to-power board is likely…
A glib answer is arduino MKR or Raspberry Pi.
For a more appropriate answer you need to supply information about your level of expertise and what you want to connect to the internet.