I’ve gone through a lot of study bibles and I think the best is the ESV Study Bible. Granted, it’s a ESV not NKJV translation but I think its the best single volume bible study resource on the market today.
For after that in the wisdom of God the world by wisdom knew not God, it pleased God by the foolishness of preaching to save them that believe. ICo 1:21 KJV
For since, in the wisdom of God, the world through wisdom did not know God, it pleased God through the foolishness of the message preached to save those whose who believe. ICo 1:21 NKJV
KJV - foolishness of the method
NKJV - foolishness of the message
Most people think the NKJV is merely the KJV modernized. It is not.