In FFVII, the few mentions of Zack and Aerith gives off the impression that they were a 'puppy love' kind of deal, and at one point she even says she's over him - but after playing Crisis Core, that didn't seem like the case to me and it's apparent how much Zack's death (or 'disappearance', because apparently no one ever told her he died) actually affected her. The first thing was obviously how she was initially drawn to Cloud because he reminded her so much of Zack, but also the fact that he gave her the iconic ribbon she's still wearing, he's the reason why she's selling flowers in the first place, and because of this "silly" request
Zack: Hey, Aerith. From now on, why don't we make a promise everytime we meet?
Aerith: Promise? For when to meet next time?
Zack: No, it's more of "general" promise about meeting. For example, when we meet, you always have to wear pink.
Aerith: Heh! Don't be silly.
she permanently changes her outfit and continues to wear pink all throughout FFVII (over four years later), since she still hopes he's still out there somewhere and she'll get to see him again one day.
The ending of Crisis Core was just heartbreaking. Zack was so close to home - he was literally gunned down right outside Midgar trying to get back to her, after reading the 89th letter she sent him over the four years he's been missing. And during the last battle
But not to focus all on the sad parts, because that was definitely not why they were a great couple; they were just so damn cute together. I loved that they conveyed such a genuine and sweet relationship without having to say it explicitly - I don't think they ever had to say "I love you", nor did they ever hug or kiss (not counting the time Aerith held Zack after his mentor died), but you can tell how much they cared for each other. It wasn't too sappy either, it just felt real, like two teenagers that would always tease each other and hang out whenever they could.
Their dates were all so adorable. I especially liked how a 1st Class Soldier took the time to build not one, not two but three flower carts just because the other two 'weren't quite good enough' for her (I think it was optional to build the better carts, but it's cannon to me. He built three flower carts).