"Thank You." It carries alot more weight than "I love you" IMO.
This is because for me, the Tidus/Yuna romance is to me, a secondary characteristic to their relationship. I find that FF fans tend to catagorize their relationship into simple cliches. To me, they are two people who are fated to cross paths, who learn and grow from eachother with the romance only being a supplement to this. Of course, there is the romantic attraction, be all means - but the depth of their relationship stems from mutual respect, admiration and eventual understanding.
By meeting Tidus, Yuna's personality opens up, and he teaches her how to look on the brighter side of life despite how dreary things maybe. In turn, Yuna's personality rubs off on Tidus - he matures, and becomes more thoughtful. Her stories of Jecht slowly make him realize that there was a side to his father that he never considered. as he was blinded by contempt. This aspect was very important to Tidus' story.
FFX was tragic, yes, but it was a bittersweet, kind of happy tragic. "I Love You" is more cliche, and it makes it seem like Tidus/Yuna were a typical Hollywood romance. This isn't the case at all. Tidus teaches Yuna to be hopeful and optimistic for the future, something she just wasn't able to do before what with her fate as a summoner and all. For that, she thanks him. He's gone, but everything she learned from him she will keep, forever in her heart.
And thus, "Thank you" IMO carries alot more meaning than the simplistic "I love you."
I know it's a bit of a repeated issue in the FFX-discussion world, but I really love this thread and hope more people reply