Mine is the 6th book, Lord of Chaos.
It starts slow but builds quickly. This is the book where Rand truly has to deal with Aes Sedai other then Morraine. He makes a strong stand and sees them as I see most AS at that point as arrogant and ignorant.
In the first 3 books they hold themselves above and separate from everyone else and the reader believes it because they use the one power, they are seen by most as almost god-like. But by book 6 we have been introduced to Aiel Wise Ones, Sea Fold Windfinders, and have had PoV from various forsaken so we know how ignorant and misinformed the AS really are.
I think Rand can see this too so he stops fearing them and seeing them as a threat.
And of course the battle at Dumi's Wells is simply amazing. In chapter 2 and 3 Taim comes to Rand and begins teaching the Ashaman. But by chapter 55 we see how fast they have become a deadly force.
It's funny though how after this book there is very little written about whats happening in the Black Tower. We only get snippets here and there from now on. Only enough to wet my appetite for more, and give me an impression that something dark is going on there.
So for these reasons Lord or Chaos is my favorite followed closely by The Shadow Rising. The story of the Aiel as told through Rand's ancestors and Perrin returning to the Two Rivers to defend his home. In TSR Perrin really comes into his own and becomes a character that I really love to this day.