Arena. Arena. Arena. I'd play it multiple times on each playthrough, partially for money and partially because it was so damn fun (except for the bandits. I hated facing those bandits...)
I also liked the Twinblade questline. Being able to sneak into the camp, steal some bandit clothes, and virtually blend in seemed amazing. But then I'd just slaughter everyone for S&G.
And, of course, the obligatory Jack of Blades battles. They were so epic and so fun to learn how to beat.
I love Fable.