It was a good finals because both contestant teams were neck to neck.
Is this sentence OK? No. A final is the last game or contest. By definition, it is singular. The word 'contestant' seems redundant. 'Neck to neck' refers to a race, usually a horse race.
Perhaps 'It was a good final because both teams went he
Not if you already know. Perhaps the previous sentence in the conversation was 'I remember the last game of the World Cup in 2002.' You don't always have to squeeze everything into one sentence.
I think when you have a best-of-seven set up like the NBA finals, you can and should use the plural. In the World Cup, there is only one final, but for the NBA, there are at least four games, usually five or six, and sometimes seven that together comprise the finals.