Maybe it was his acrobatic, one-handed interception back in Week 9 against Dallas, when he leaped high in front of receiver Miles Austin to haul in Jon Kitna's deep throw.
Or maybe it was in the Wild Card playoff game in Philadelphia, when he was stride-for-stride with speedster Jeremy Maclin on a go route, kept his concentration, and managed to knock the ball out of Maclin's hands at just the right moment.
But if neither of those works, Sunday's NFC Championship certainly does.
Playing on a bigger stage than the former college receiver ever could have envisioned in his first year in the NFL, Shields had a sack, a forced fumble and two interceptions, including the game-clinching pick at the Green Bay 12-yard line with 37 seconds left to seal the Packers' 21-14 win and a trip to Super Bowl XLV.
"The guy played lights out," linebacker Desmond Bishop said.
Shields was all over Soldier Field, blitzing from the edge and covering the speedy Johnny Knox much of the day.
His first big play was right at the 2-minute warning of the first half. The Bears faced third-and-7 on their own 33 when Shields blitzed and sacked quarterback Jay Cutler for an 8-yard loss. He also knocked the ball out of Cutler's hands, but Chicago running back Matt Forte recovered the fumble.
Then, on Chicago's next possession right before halftime, Cutler tried to go up top to Knox (two catches, 56 yards) from the Green Bay 41, but Shields was in perfect position. He jumped up and brought the ball in with both hands, tumbling to the ground at the 3-yard line. Upheld on a replay review, the interception kept the Packers' lead at 14-0 at intermission.
"It was definitely big," fellow cornerback Tramon Williams said. "Guys understand the situation we are in. Chicago was about at midfield at that point and I know Sam knew that when a team usually gets the ball at that point, guys like to take shots and kind of get big plays off it. He played his technique and did a great job."
But Shields wasn't done.
In the fourth quarter, he was called on to blitz again on third down, forcing third-string quarterback Caleb Hanie to go to his hot read, Forte out of the backfield. Only nose tackle B.J. Raji had dropped into coverage on Shields' blitz, right in Forte's path, and Raji picked the ball off and cruised 18 yards into the end zone for a touchdown and a 21-7 lead.
Then came the game-saver. After Hanie got the Bears back within a touchdown at 21-14, he drove them inside the Green Bay 30 in the final minute.
But on fourth-and-5, he tried to go deep down the left side to Knox, who was double-covered. Shields had inside leverage on Knox and made the interception at the 12, clinching the win.
"I was underneath and undercut the receiver and the ball was there and I attacked it," Shields said.
He also took off to return the ball, despite pleas from his teammates and everyone on the sideline to just go down. He hit the deck after a 32-yard return, and victory was assured.
All this from a youngster who played just one college season at cornerback, didn't get drafted, and then not only made the Green Bay roster as a rookie free agent but earned the starting nickel job for the season opener and hasn't given it up.
"Unbelievable," safety Nick Collins said. "The guy just worked hard, listened to his coach, listened to the veterans, and it paid off. I'm just glad he was able to have so much success with us, coming from Miami, only playing 10 games at corner and really not knowing how to read an offense. Man, this year he played phenomenal."
General Manager Ted Thompson said after the game the Packers were "very, very lucky" to get him signed right after the draft, and he praised the work done by cornerbacks coach Joe Whitt Jr. in tutoring Shields all season long.
Veteran Charles Woodson, whom Shields referred to earlier this week as his "big brother," had a hand in that tutelage too.
"One thing we knew about Sam is we knew he'd be able to play at some point, but we didn't know how fast he'd come around," Woodson said. "One thing we knew about him early on is he had no fear. The guy was confident in his abilities. He came in and got noticed early, and we knew at some point he'd be able to play for this team. He's come on better than anybody could have expected."
That includes the coaching staff, though defensive coordinator Dom Capers noted that Shields' performance on Sunday was typical of how various players have come up big on defense at critical times. Of late, it's been unheralded outside linebacker Erik Walden winning NFC Defensive Player of the Week in the regular-season finale and Williams snagging three interceptions (including one returned for a touchdown) in the first two playoff games.
"That's what happens," Capers said. "The last two playoff games, Tramon's been tremendous and they've thrown over there. Now, probably they didn't want to throw over there. They threw at Sam, and Sam made a couple big ones. So to me, that's the sign of a good defense because different guys are going to make plays at different times, and they can't just zero in on one person."