There's so many factors (rule changes etc) it's really impossible to name a number 1. And I can't take a list like this seriously without opponent strength being factored. You also have to factor in how good their offense was, I'd give extra points to a good "D" who had a lousy offense.
The 2014 Seahawks faced 8 offenses that ranked 21-32 in the regular season, and only 4 teams ranked in the top ten. In the 8 games where they had to face a reasonable offense (ranked in the top 20) they allowed a mortal 20 PPG average.
The 1985 Bears is probably the best I've ever seen - but they caught the league off guard with the "46" D and they had a meltdown against Dan Marino.
The 1976 Steelers is also up there on my list. They have some of the best numbers even though they started their season 1-4. At that point Joe Green actually got pissed and told his team "From here on nobody gets past the 50 yard line!" They proceeded to allow only 28 points over their remaining 9 games and completely shut-out 5 teams! But they slipped against the Raiders in the playoffs.
I don't see the 86 Giants listed, and you "hated" putting the 1990 team on the board? They were one of the best I've seen, playing Joe Montana twice and allowing only 20 points. Those were some of the best games I've ever seen, finesse Hall of Famers vs Brutal Killers. The Giants hit Montana so hard it symbolically ended his career with the Niners, and they also gave him a nasty concussion in 1986. In the Super Bowl they beat the high flying Bills no-huddle offense who were just rolling over teams (they beat the Raiders 51-3 in the Championship game.) The Giants had to switch to nickel & dime packages designed by defensive coordinator Bill Belichick and won the game 20-19.
The 1986 team was just as nasty. They harassed QB's while only bringing four bodies (DL's Marshall, Martin & Burt with "LT" flying in from all over the place.) The other 3 LB's were top notch - Carl Banks who shut down TE's while Carson & Reasons stuffed runs up the middle and they had a solid rookie in Pepper Johnson. They used a "bend but don't break" soft 2-deep zone that didn't allow any big plays. With the LB's speed you couldn't throw to the flats or run outside, and you couldn't run inside either against Burt, Carson & Reasons. All you could do was throw little underneath stuff in the middle of the field...and promptly get blasted. Thus, they were a WR's nightmare.
In 1986 they beat Bill Walsh's Niners twice, John Elway twice, the Eagles Ron Jaworski/Randall Cunningham twice, Neil Lomax twice, Dan Fouts, Jim Plunkett and the Vikings Tommy Kramer. They also beat the Redskins 3 times against 4000 yard passer Jay Shroeder who had the hogs & one of the best receiving duo's ever (Monk & Clark.) They should've been led by Joe Theismann, but...well the Giants broke his leg in 1985. The only ones who beat them was the Cowboys Danny White (who they beat & knocked out in the 2nd game) and Seahawks Dave Kreig. That's crazy, I mean how many Hall of Famers did this team face?
Imagine having to go up against the 49ers Roger Craig, Jerry Rice & Dwight Clark? San Diego's Joiner, Chandler & Winslow? The Redskins Monk & Clark? Dallas had WR Tony Hill along with Herschel Walker & Tony Dorset in the backfield. The Raiders had Marcus Allen, Seattle had RB Curt Warner & WR Steve Largent. I'd like to see some of the other mentionable defenses play and beat those offenses.
And just for the record I'm a Niners fan. The 80's Giants were just scary man, I don't think any team from any era would want to face them. I think the 85 & 86 team knocked 9 QB's out of the lineup which included Joe Theismann, Tommy Kramer, Ron Jaworski, Danny White and Joe Montana.