I'd say the episodes in the series are,
Mermaid Man and Barnacle Boy I-VI (Of course)
Mermaid Man VS Spongebob
Back to the Past
The Bad Guy Club For Villains
Mermaid Man Begins
& Super Evil Aquatic Villain Team Up Is GO!
I'd say The Bad Guy Club For Villains is my favorite, since it's a funny episode similar to MM&BB V, except with more action.
As for the worst, I would say The Card, but MM&BB don't really play a role in that episode at all. So I'll go with...