As well as the ones you've mentioned, Roy Jones Jr was almost unhittable in his prime, although that was down to superhuman reflexes more than traditional defensive skills. Hopkins has a very crafty defense, and James Toney did some great work defensively. He was hittable, but very rarely got caught flush when he was on form. His performance against Jirov was astounding - for an old, fat and seemingly washed up super-middleweight to fight off the ropes against a murderous, volume punching cruiserweight like Vassily Jirov should have been suicide, but his defensive skills, counterpunching and chin wore him down and won Toney the fight.
He's not on the level of those guys and I hated watching him fight but Winky Wright had one of the more effective defenses I've seen lately. Very high guard, caught most punches with his gloves.