Season 1: Owen. He was the first winner who's ending was made years prior to Gwen's victory.
Season 2: Beth. She is considered the canon winner since the CMTDARS Special show her and Lindsay in France. It can be implied that Beth was also the canon winner for Total Drama Action since a scene from that said special show her with a wheelbarrow of money, likely her million dollar prize, ready to bail her and Lindsay out.
Season 3: Alejandro. He is considered the canon winner of Total Drama World Tour since a scene showed that Alejandro was concerned about the money, by telling Chris: "Chris, the million dollars. Is it safe?". It is implied that Alejandro is the real winner for TDWT, even though his ending was made after Heather's. He is the canon winner since Heather was in normal condition during her TDRI cameo in episode 10.
Season 4: Cameron/Lightning. Wait until the TDRI special to see which ending they used.