My favorite class in the game is still the first I picked for my MC on day one: Chanter!
I adore the innovative - if a bit flawed - system of the class. The phrases and the invocations are FULL of flavor and they have some of the best skills in the game. Also, it's fantastic that the game lets me change the .ogg audio files freely, so I can make my characters sing whatever I want them to!
That's an awesome feature that I never had the chance to experience before and for sure made Chanters special in my heart.
Some of the names+artworks of their skills are among my favorite in the whole game, especially At the Sound of His Voice, the Killers Froze Still, The Lover Cried Out To The Beloved: "I am Yours!". You should hear them cast with my custom sounds!
I can never play without a Chanter in my team, from the first time up to my POTD run. I'll soon be doing Triple Crown, with a Chanter of course!
Ah, I could talk about them for hours, sorry if I'm being verbose!
My least favorite class is Rogue, sadly. I like the concept of the class, so it's probably my fault, because I'm a total inept at playing Rogue, and I've always felt that the melee assassin was somewhat lacking, especially in higher difficulties. It probably depends on the fact that I like to play them as glass cannons... frustrating experience! I read that the class got a recent buff, I might give them another try.