This poll is missing the point. The question is not what AC the other party members have, but what attack bonus the monsters have that you fight.
The game is balanced around the idea that the standard monster hits the standard non-defender PC of the same level with a chance of 50%.
If you look at the monster creation guidelines, this sets average AC for all PCs at 15+level. This value more or less holds true from level 1 to 30. Everything above that is good AC, anything below that is bad AC.
Example: The group I DM'ed had a 10th-level sorcerer with AC 19. Even worse, that guy had no sense for personal security and always tried to attack stuff with Sorcerous Blade Channeling in melee. Level 10 monsters could roll ridiculously low and still hit him. The PC was always bloodied in one or two rounds, and it's a fricking miracle (and graciousness on the DM side) that he only went down once.
The 10th-level Genasi Swordmage in the same group had AC 28, which is standard defender AC for that level. It's not that the AC span between him and the sorcerer was too wide, the sorcerer's AC was just ridiculously bad.
Caveat 1: The DM can adjust standard monster level up or down one or two depending on how well-optimized the party is. But this means monster damage, defenses and general nastiness also go up or down. It's not possible to only raise or lower monster attack ratings for one character whose AC is off the charts up or down.
Caveat 2: Some classes have innate defensive abilities that are calculated in. I'm talking about the Warlock's Shadow Walk etc.
Caveat 3: Monster melee attacks target AC more often, while monster ranged attacks target NADs more often. That is why AC is somewhat more important for melee characters, while ranged combatants can focus on NADs.
Caveat 4: Defenders are expected to invite monster attacks, but shake them off more easily. That is why their AC should be better than average, but not at unhittable levels. The sweet spot is between 16+level and 20+level.
Caveat 5: AC is not the only important figure in survivability. Also consider NADs, the ability to generate temp hp, better/more saves, self-healing, concealment/invisibility etc.
Caveat 6: Strikers are a magnet for monster attacks. Striker AC can never be high enough.
Caveat 7: When you consider raising AC, remember that everything has an opportunity cost. The feats, powers and magic items you spend on raising your AC cannot be spend on offense, mobility, and other defenses.