The most impressive thing I've ever seen was Brian Urlacher in the "They are who we thought they are" game against Denny Green's Cardinals on MNF. The unbeaten Bears were getting destroyed by Denny's Cardinals. The Cardinals forced six turnovers, did not allow Chicago to score on offense, up by 20-0 at halftime, and controlled the ball for almost 40 minutes. However, Brian Urlacher was possessed when he came out of the locker room at halftime. His stats mainly show 19 tackles, but he had numerous pass breakups and tackles for loss plus the huge forced fumble that was returned for a touchdown.
Hands down it was the most impressive one-man show that I've seen in my entire life. I wish that I had that entire game on video somewhere.
My second-place vote is a homer moment, but it was seeing Edgar Bennett make a full layout dive along the left sideline for a Brett Favre pass back in 1995 or 1996. I still have yet to see a running back make that kind of play on a reception. I'm still impressed.