Season 3. We get the most of Oz and he has a lot of great moments And episodes. Beauty and the Beasts, Amends, Choices, Graduation Day. I love Oz especially Oz and Willow but I also like him and Xanders interactions. I just miss Oz when he leaves in s4.
Season 3 because he gets more focus in some of the episodes.
He's consistantly funnier than Xander this season, and also more likeable but he gets some good, dramatic moments. His reaction to events in 'B&TB', his breakup/reconciliation with Willow, and his intense actions in 'Choices'- everybody is arguing about whether to exchange the box for Willow and Oz totally 'gordian knot's the situation to settle the argument.
I did like him a lot in S4. I think if he'd gotten a full run of episodes there it would've been his best season.