I've looked at some numbers, and the quantitative case for Hester is surprisingly strong.
As a starting point: how much is any single player, who is not a quarterback, worth to his team? How much would the spread move if he was out? The answers that I've seen mostly max out around a point or two, maybe 2.5.
This article from 2013 has 7 non-QBs worth 1 point or more, led by Adrian Peterson at 2.5 points and Calvin Johnson at 1.5 (it further claims "no non-quarterback has been more valuable [than Peterson] in the last 25 years — with only Barry Sanders equaling Peterson’s point spread effect").
This article from 2016 claims that the most valuable non-QBs are only worth 0.5-1.0 points, and that only about 6 players are even worth that much.
This article says the spread moved 2 points (from 4 to 6) when Adrian Peterson was out and
this one says it moved 1 point (from 6 to 7) when Gronk was out.
ProFootballLogic has estimates for all players for the 2017 season and lists 6 non-QBs at 1 point or more, led by Gronk at 1.63 and Antonio Brown at 1.21.
Now, how many points per game was Hester worth? Let's focus on the 6 consecutive seasons 2006-2011, which covers his prime (though he had some ups and downs in there).
Football Outsiders special teams stats, averaging across those 6 years, have the Bears punt return unit worth 10.56 points above average, their kick return unit worth 11.16 points above average, and their "hidden" special teams value worth 12.45 points above average. "HIDDEN represents the advantage teams have received from elements of special teams generally out of their control: opposing field goals, kickoff distance, and punt distance" - for these Bears teams that was probably mostly due to how other teams game-planned for their return units. Add those three numbers up and the 2006-11 Bears return units were worth 34.2 points per season above the average team, or 2.1 points per game. That's more than the value of just about any single non-quarterback player, with possible exceptions for Adrian Peterson and Barry Sanders in their prime.
Devin Hester wasn't responsible for all of that - he shared kick return duties with Danieal Manning and Johnny Knox, and the entire Bears special teams were really strong under Dave Toub. If we give Hester credit for 3/4 of the team's punt return value, 1/3 of the kick return value, and 2/3 of the hidden value, that would make Hester worth 1.25 points per game above the average returner. Which would make him the 6th, 1st, or 2nd most valuable non-QB in the NFL during a season, if we buy one of the three sources of player point spread value linked above. (Actually, it might make him worth a little more than that, since these Hester stats compare him to the average starting returner and the spread values compare players to their replacement-level backup.)
Is 1.25 points per game of value from Hester plausible? Here's another way to look at it. Hester had 19 return TDs during those 6 years (12 regular season punt returns, 5 regular season kickoff returns, 1 regular season missed field goal return, and 1 playoff kickoff return). So he scored 114 points in 97 games. But a typical return TD is worth more like 5 points to the offense (since they're using up their possession to get the score), and an average unit would've gotten about 4 TDs with the same number of returns, and let's say that an average returner with the Bears special teams would have gotten 6. So that leaves Hester's TDs as being worth something like 0.67 points per game more than what the Bears would've gotten with an average returner (or 0.77 more than what an average return team would've done).
0.67 is just the touchdowns, not the field position from the returns and the non-returns. And the field position gain seems pretty big too. Take a look at
this 2007 Bears-Eagles game, for instance. The Eagles kicked off 5 times and punted 4 times. Devin Hester was not able to return any of those 9 kicks, but he had a huge impact on the game, worth something like 75 yards of field position. After the 5 kickoffs, the Bears' average starting field position was their own 34: one KO went out of bounds and the other 4 were kicked short and fielded by the up-man before they reached the CHI 20 yard-line. The Eagles' 4 punts went for 21 yards (no return), 41 yards (touchback), 27 yards (no return), and 43 yards (no return, inside 20), with an average of 28.0 yards net (33.0 yards gross) and only 1 ending up inside the 20. A typical
punt nets around 38.5 yards, and typical
starting field position after a kickoff is around a team's own 27, so kicking away from Hester cost the Eagles something like 40 yards of field position on punts and 35 yards of field position on kickoffs. The Bears offense was able to generate 5 field goal attempts and 1 touchdown, and they won the game 19-16.
That's just one game, but it's hardly unique. The Bears led the league in best
average starting field position in 4 out of those 6 seasons, on average starting a possession 2.4 yards ahead of the average team (which is a 29 yard advantage each game - probably worth more than 1 point). Devin Hester played a large role in that, and his touchdowns don't count towards it at all (though of course the defense and the rest of the special teams also played significant roles).
So 1.25 points of value per game from Hester seems plausible to me, based on a combination of the flashy plays and the sacrifices that other teams made to avoid those flashy plays (and ignoring his contributions on offense). That would put him in pretty exclusive territory, for non-QBs, according to all of the sources that I've found on that topic.
There are also some knocks on Hester: this is only a 6-year stretch (his other 4 seasons as a primary returner were nothing spectacular), he fumbled a lot, Toub and the rest of the Bears' return units deserve a good chunk of credit. But I don't think his HoF case is easy to dismiss, even just focusing on the numbers.
