Board Thread:Questions and Answers/@comment-2A02:A03F:4AF5:4A00:2448:9F50:F890:58C0-20181001185417/@comment-6697950-20181001190158

Season 1 --> Extra Edition (optional) --> Season 2 --> Ordinal Scale --> SAO: Alicization.

Add Sword Art Online Alternative "Gun Gale Online" after season 2 if you want to watch a spin off. It doesn't really have much of any ties with the story of the main series though. And it has an original cast.