Personally I always prefer ESP32 for my embedded prototype development.
That's because, it has inbuilt Wifi, BLE and classic Bluetooth along with multiple I2C peripheral with many more features. It is far better than arduino for prototyping purposes. It has high clock, higher pin count. I like this module not only as an evaluation board perspective but also in the easiness of designing PCB's with this SoC. I say it is one of the best developed SoC's available in the market. It is lot easier and faster to get the products into market with this SoC.
I don't think so there is an ideal Embedded development board. Every development board has it's own pros and cons. But,
- For beginners - Arduino, ESP32, Raspberry Pi can be good development boards
2. For Firmware development enthusiasts - NXP FRDMKL25Z Freedom board, STM32 Nucleo Board (There are many more but I just got these to my head)
All these points are just made from my perspective. Feel free to add up or correct my points...!!