Before "The Replacement". Season 5 episode 3, Xander might as well have had "LOSER" tattooed across his forehead. He got split into two people, one successful and one that was purified loser and when he was recombined, he remained successful Xander, possibly instant karma from blocking a shot from Toth that was intended for Buffy. By season 7, he had a successful job as a building contractor and he was finally married to Dawn in the comics and I approve of this ship - it started from a position of mutual respect. Season 5 was the transformation so I guess that is may favourite season for Xander.