I feel similar to Dark Witch. I think Alder has the best character development and Iris is the best champion. I like the maturity and empathy in his character, and wisdom he's acquired from the tough times he's been through, even if there are other champions who are stronger. I think of him as mentor or father, especially in BW2.
For the battle itself though I would vote for Iris, there's nothing like duelling dragons, and hers are strong and varried-- its a dazzling team! Her theme is my favorite for a champion. Usually I am feeling serious by end game, but as soon as I heard that music I thought to myself-- gawd I'm about to have so much fun. Her attire is beautiful, and she's just really fun-- especially since she's so impish and doesn't accept defeat with as much reserve as others. She's just really amusing!