I'm going to be fighting all of these guys again soon, so here's my old thoughts, from easiest to hardest.
1.Phantom
2.Xemnas
3.Kurt Zisa
4.Ice Titan
5.Sephiroth
Phantom is the easiest to me because I can take him on at Lv50 just fine. I always go for him right after finishing Hollow Bastion so I don't miss out on any Clock Tower rewards. It takes 20 minutes to beat him, but with Sora on a magic-inclined setup along with Goofy/Peter Pan dealing physical damage, he's not that bad. Just time-consuming.
With Xemnas, he kicked my ass the first time I fought him, but now, he's kind of a pushover. The only time I'm ever in trouble is if he locks up my controls, but other than that, I have no problems during that fight. I'm only ranking him higher than Phantom because when I fight Xemnas I'm usually 30 levels higher than the Phantom, so I feel like that has an impact on my experiences. I know he gave me a hard time during my first playthrough, but I can't remember why.
Kurt Zisa is more of tedious than anything else. He only gets dangerous when he starts using his horizontal and vertical blade attacks which I can dodge alright, but can be a bit tricky. This is the time when I decide to break out Tinker Bell right before he seals off my magic, just in case something goes wrong. That one part can be annoying, and almost killed me at times, but other than that, I don't think he's too bad.
Ice Titan is a little trickier for me. The main reason why I have a hard time even now is because due to Dodge Roll not having nearly as many invincibility frames as the later games, I have a habit of jumping instead of Dodge Rolling order to dodge attacks. Unfortunately, that does not work well with Ice Titan since he drops ice blocks directly above you. I haven't died against this fight since my first playthrough, but until I break that habit this is a fight that continues to annoy me.
Sephiroth is definitely the hardest fight for me. I still don't have his patterns down perfectly, so that's one problem. The main reason why I rank him at No.1 is because of Heartless Angel. He does give a warning when he uses it, and thankfully Sliding Dash in Final Mix makes it much easier to reach him, but there's always that one point where I don't expect him to use it, and he does. Sometimes I manage to get that Elixir in, sometimes I don't.