While readily available porn has taken away the "wow factor" somewhat, nothing beats real live experience - even if it's not actual sex it's still crazy exciting!
In my case, it was when my wife and I were dating and we walked in on my roommate as he was exiting the shower, thinking no one else was home. Towel over head, and 7" thick inches, soft and swinging. Wife saw it and audibly gasped. Believe he had about 9" hard. When we had sex later the wife was the wettest I ever felt her, no doubt wishing the thin 5.5" I was poking her Asian pussy with was the thick 9" she had seen earlier!
What's the biggest cock your wife has seen in the flesh in real life?