Without a doubt, the best mission of the entire first book is the ninth one - "Green Wave".
Firstly, it adds this nice felling of usefulness for dwarves and elves, as they aren't some "allies that help the humans", but they on their own have their strength, and can deal with some hard problems ON THEIR OWN, without those smelly humans. It feels nice to control brand new race, dwarves, with their own units, and new models (just for this one mission!) so the player feels some fresh difference. Also, it's giving the feel that the world is alive, because we are not only dealing with the problems that our "main" character is having trouble with (Blen, because he is somehow a main character of this book), but there are stuff happening outside of the "main" story, with different people having their own battles. Also the design that our bases are connected with eachother and are one, big base is really cool.
Secondly, by gameplay, it's (at least for me) the hardest mission of this book (I played it on hard, too), and it makes sense with the story WHY it's so hard - as was explained at the end of it, you were facing five (I think?) orc clans at the same time, and it's not like they attack one after another, but sometimes they attack simultaneously, which makes it really hard to comprehend what is happening, especially with the one clan attacking you by air.
Also, what is adding a nice feel for the mission is that the clans are not attacking with the same boring, units, but every each one of them have some special units, that other clans do not. Blue attacks with Hero (Ralf, because "Dwarves and Elves? They are weaklings, get some humans in here!"), red with champions, purple with shamans, green with air units, yellow with raiders (although I think he shares them with dark green, if I'm not wrong?). It adds a nice feel that you are not facing five identical clans, but five different clans, that specialize on other units than everyone else. Red is strong, so they have the strongest orcish units - champions; purple mains magic, so they have shamans, etc.
So, yeah, I really like this mission. Also because I feel this entire book is filled with not really interesting ones, except this one, in comparison to the orc first book and undead first book.