I love both Zack & Cloud because of their friendship , especialy when Zack helped Cloud from Hojo's experiment ,he didn't abandoned Cloud.
On the other hand , he gived it his best shot to protected his best friend from Shinra troops.
At last he knowed he would died and so sad for couldn't met Aerith again.
Nevertheless he know his best friend still alive , he could smiled and sent his Living Legacy to Cloud.
In my opinion this was the best scene of FFVII , it looked like husband asked wife to lived in spite of without him.
I confirm that relatinship between Zack & Cloud certainly was more than friends.