But when we talk about 'weakest'... and 'strongest'... where are the standards for that? I mean, like it was already said, Sesshoumaru is already pretty darn strong. Could an enemy defeat him if the moon is a new moon? I don't like the idea of a Sesshoumaru that could lose in a fight.
Or perhaps... that would make a good plot?

Him overcoming the weaknesses that nature bestowed on him sounds like something we can write about!
What about his control over his beast? His control is weakest at the full moon, and strongest at the new moon. Kinda like his beast is loose one night per month... So, we can see him losing control to his beast on a full moon, but completely composed on the new moon.
That would work. I guess.
And some powers... Hmmmm. Anyone has ideas about new powers?
Edit: I'm kinda repeating myself, aren't I? n_n'