I went to supply depot just to compare armor at their base stats.

At 4*, all armor have downsides that negates the upsides.

If we use cobalt as benchmark.

4* wolver line:
=armor def about same as cobalt
+ freeze def
+ DB
(cobalt + 2 positives)

4* heavy line:
+armor def higher than cobalt
-no second armor def
+stun def
-sleep deb
-ASD
(2 positives, 3 negatives)

4* mercurial line:
=armor def about same as cobalt, except flipped normal-pierce
+electric def
(cobalt + 1 positive)
(personally i'd say reduced normal def is a negative since most damage are normal, but for sake of brevity)

4* skelly line & magic cloak line:
=armor def about same as cobalt, except flipped normal-darkness / normal-elemental
+poison def / fire def
+freeze def / shock def
(cobalt + 2 positives)

4* angel line:
-armor def lower than cobalt
+poison
-fire
+curse
+fiend
(cobalt +1)

4* virulisk:
=armor def about same as cobalt
+poison
+slime
(cobalt +2)

4* salamander: I don't even know what its for

4* wyvern:
=armor def about same as cobalt
-no normal armor def
+poison
+fire
+beast
(cobalt +2)

TLDR:
-almost all armor outclass cobalt, except a few.
-heavy is underpowered
-virulisk/wyvern/salamander tradeoff for its limited functionality seems not worth it
-generally good armors are wolver, magic cloak, and mercurial (with zero downsides, which I feel kinda breaks the balance)

IMO:
-heavy need buff, maybe sword BD
-cobalt need small buff, maybe +1 health
-this might make people angry, but wolver, and magic cloak should get 1 poison/fire/stun/.. debuff