I'm honestly not sure I understand the question... Then again I don't watch anime, so if it's something related to that it makes sense I don't get it.
(I do know what arcs are, sort of another name for 'seasons' of shows and such, just not what you mean in this context)
Well, if you're asking about what my favorite arc is in general, *shrugs*. If you mean favorite part of a specific show, I'd say the second half of Transformers Armada, though there are many I could consider as alternatives just because of how much I love good storytelling.
Have you ever been to the top of a lighthouse? (IRL)