Well, if we're talking main series, probably Talons of Power. It really gives you that feeling of suspense, and is the absolute perfect penultimate chapter for the second Arc.
If we're talking about just any WoF book on a shelf though, I'd have to go Darkstalker, no competition. I've read Darkstalker countless times more than the other books. Sutherland really outdid herself on this story, and I'd wish she'd do more. Not only does it give you more world-building and a glance into the normal, everyday lives of some of the Dragons on this continent, it also has some serious, beautiful themes to go along with it.
There are so many topics that the Darkstalker book can bring to the table, and in my eyes, it's one of the few books being produced for this demographic that can really make you stop and think about things, and prevent you from looking at things the same way again. There are also some really interesting psychological aspects of it that fascinate me, such as Albatross's road to insanity. It shows there's more to Animus magic that meets the eye. While Darkstalker is the main antagonist of the book, and the second arc as a whole, the Darkstalker book really prevents you from just seeing him as a power-hungry, conniving villain. You read this book, and when you are actually in his PoV, everything makes perfect sense, and it seems to you that what he's about to do is the right thing to do, and then you shift over to someone else's perspective and realise "Oh my gosh, what the heck did he just do??" And even towards the beginning of the book, he's a legitimately good Dragon, but a few significantly placed events, such as Queen Vigilance's attempted assassination of him, leads him down the wrong path towards the end. The book really helps you view him as the tragic hero he's made out to be, which is also why I absolutely hate how Darkness of Dragons ends.
Clearsight's PoV is also truly fascinating, and she's really the character from this book I most identify with. She's calm, logical, and grounded in reality, and for part of the book she keeps Darkstalker in check until he goes down the wrong path towards the end, where we see she has a fiercer side as well. She also gives a fascinating look into the mind of a NightWing seer, and a look into NightWing culture as a whole, which we don't get to see very often.
Fathom's perspective gives us some much needed humility, as well as a character who is very cautious and at the same time is arguably the most impactful character in the book. Not only does he actually create the spell to stop Darkstalker, he also is the one who discovers the spells in invisible ink. This does not mean that Fathom is all good, however, as his arrival and his tale of the SeaWing massacre is one of the things that drives Darkstalker to cast that all important spell on his scales that sends everything downhill.
So, that's my opinion, Darkstalker is my fave book, and if you actually read this whole thing, thank you.