Hm, let's start from the beginning.
Cloud wasn't being himself in the beginning of the game, and at that time he met Aerith who saw "the Zack" in Cloud, and fell for it. She suddenly started to flirt with him (wanted to have the first conversation Zack and she had), and Cloud played along because he acted as someone else.. Yes, Aerith's dead boyfriend
Zack. Later in the game when you can get Aerith (or Tifa, Yuffie, Barret) on a date, she says: "I want to meet the REAL you.." And I think she started to feel that Cloud wasn't being himself. (Aerith has the ability to see through Cloud's troubled mind, because she is a Cetra.) But Cloud at the time, didn't really get it..
So when Aerith dies Cloud is shocked. Of course! He promised to be her bodyguard, but wasn't able to keep that promise. Cloud always wants to protect something dear to him, am I right? So he felt guilt.. But at the same time Cloud still wasn't being himself, so Aerith never got to know the REAL Cloud. And the REAL Cloud is a totally different person. Other memories, other feelings etc.
And then.. we come to the part were Cloud and Tifa falls down into the Lifestream. Tifa starts to build up the real Cloud together with himself, and they came to realize their feelings for eachother. Cloud had secrets, Tifa had secrets. But now they knew, and after that they got very close.. (Personally I think so.) Cloud held Tifa's hand to keep her safe, and they slept together in a kind of.. intimate position under the Highwind.
And then in the end as well.. Cloud is in the Lifestream, and Aerith reaches her hand to him. But before they "touched" eachother, Tifa reaches her hand to Cloud.. And he kind of "wakes up"? XDD And the rest is pretty obvious that these two have something together, Cloud catches Tifa when she's about to fall down, and then she rests her head on his chest.
Okay, that was the game.. NOW, AC.~
Two years have passed, Cloud wanted to start a new life with Tifa.. And they did.
Marlene and Denzel became a part of their family as well, and they were happy.
But then, Cloud got qeostiqma and all the painful memories he had left behind started to fill his mind. He realized that he could loose something he cared about again, and left Tifa and the kids. (That's a proof, from the Ultimania.) But in the end, Cloud is forgiven and returns to his family. Again, he lets go of his guilt. So the happy Cloud in the ending scene, is really Cloud. The Cloud who was "dimly" in love with Tifa, came to realize his feelings for her, wanted to start a new life with her, and the one who lost two really good friends.
So I think it's really clear wich couple that is canon, Cloti. X333
In the other hand, I think Aerith still had feelings for Cloud. And yeah..
But in the new Ultimania it says that Zack
always will be by Aerith's side, watching over Cloud and the others.
Just wanna let you know that I used to be a Clerith when I had played the game, and yes.. I still love them, but I still consider Cloti as canon. So I love both.