Interesting thread. I think either of the sentiments expressed by Yuna would be a nice way to end the game.
Having her say "Thank You" at the end of the game, it shows how much Yuna appreciates Tidus and how much he's changed her life, but by saying "I love you" it can also show just how much Yuna loves what Tidus has done for her, and it shows her own feelings towards him.
I think "Thank You" shows a future where Yuna can be happy without Tidus, while having her say "I Love you" shows that she needs Tidus to be truly happy, and for me anyway, adds a lot more emotion to the ending.
Having her say "Thank You" at the end of the game, it shows how much Yuna appreciates Tidus and how much he's changed her life, but by saying "I love you" it can also show just how much Yuna loves what Tidus has done for her, and it shows her own feelings towards him.
I think "Thank You" shows a future where Yuna can be happy without Tidus, while having her say "I Love you" shows that she needs Tidus to be truly happy, and for me anyway, adds a lot more emotion to the ending.

I'm just gonna stick with "Thank you."
I guess 'thank you' suggests that the words 'I love you' aren't needed. Yuna and Tidus already know how the other feels; the two love one another beyond words. However, I would still argue that the words 'I love you' are important. Yuna believes she won't see Tidus again... Saying those words in that final moment make them mean something.