If Devil's Claw is what the Black Omen is? That's effectively the final dungeon. Yeah, its optional due to the nature of CT's 2nd half, but its by far the longest dungeon in the game, complete with at least 4 bosses along the way, plays directly into the game's main plot (rather than an arbitrary side quest that, at most, is relevant to a single character), and has by far the toughest enemies in the game. Believe Gaspar doesn't even talk about it like any other Fated Hour quest, but rather, only mentions it saying "its connected to Lavos!"
Oh yeah, the clincher to all this is that as soon as the dungeon is done, you're shoved right in front of Lavos himself, I don't think with any choice either (mind you, once the first stage is beaten, you can actually pull out. Beating the outer shell is NOT a point of no return, odd as that sounds for a Final Boss.)
So yeah, hold off on that dungeon til last cause its very clearly meant to be done last. Also, be sure to have Magus in it for the last few fights, if purely for stylistic reasons (not only some extra dialog, but it actually goes so far as to change the music of one fight too.)