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Identifying the best run defense ever for the Whoa Nellie game was easy, but the best pass defense is trickier. 

Here's the pertinent stats:
1979 Alabama
36% completion rate
78 yards per game
25 interceptions
3.9 yards per attempt
.
1997 Michigan
50% completion rate
116 yards per game
22 interceptions
4.4 yards per attempt
.
2002 Miami
46% completion rate
120 yards per game
12 interceptions
4.4 yards per attempt
.
2011 Alabama
49% completion rate
112 yards per game
13 interceptions
4.3 yards per attempt



Tough part is you have to take into account each era.  No one passed in 1979, so they faced a bunch of option teams, yet the Tide was still especially good compared to peers.  Interceptions were very high in that era.
Passing increased a lot by 1997 and Woodson was great, obviously. 
We all know the talent on the 01-02 Canes.
Passing increased still into the current era, but still the '11 Tide's numbers stack up favorably. 

Taking all that into account, which pass defense was most impressive?

2011.

Agreed.  As @OrangeAfroMan noted in the OP, opponents were a LOT better at passing in 2011 than they were in 2002, 1997, and especially 1979 so I find Bama's 2011 pass defense more impressive because they did it against better passing opponents.  

1992 bama is not on the list?   

1992 Alabama
50% completion rate
139 yards per game
23 interceptions
5.1 yards per attempt

Good, but slightly below these.  I also looked up 71 Nebraska, 01 Miami, 81 Clemson, and 91 Washington for this, as well as late 70s/early 80s Penn State teams.  They were all great, but a slice below the poll options.

The Desert Swarm defenses were the best ever vs the run and had plenty of sacks (separate rating in the game), but were very average in pass defense.

nope

Thank you very much, guys.  I'll make this official on Whoa Nellie's facebook group.