Hard to come up with a 'most important' as any knowledge about God is very important for a believer.
But, I will say, it's most important for the believer to know that all knowledge about God comes from the Bible.
I realize some may say but there is general revelation. But I would say, we don't understand general revelation without the Bible. Or some may say, the Holy Spirit gives us knowledge of God. Which is true, but again, we don't understand that without the Bible.
I'ts God Who places this value upon His Word. (Ps. 138:2) And the Bible is the written Word of God. (Ps. 119:1-176)
I'd say that the 'most important' is knowing that Jesus is God, and everything that we might wonder about that relates to a supreme being is revealed in Him.
That Jesus is Lord and God raised Him from the dead, cause you will be saved if you believe that, but you also have to know that He loves you and that you can't live like the devil, cause demons believe that too and it doesn't save them.
For me, it's His love for me. During July, 1994, I walked into a church I knew nothing about and when I went for prayer, I got a revelation from the Father of His love for me. I had been earnestly seeking Him for 4 months. I found Him. I was a prodigal. It was the best day of my life!