For party members, I'll mirror what people have said about Aerith really standing out. Even in battle, having the staff flick magic instead of just being a low damage weapon was really nice.

I also enjoyed the extra time we were able to spend with everyone in Avalanche.

Endgame SpoilersNow that we have at least Biggs and Wedge (???) alive, I wonder if they're going to play a special role in future games. I could see them showing up in random towns, either being special item suppliers or helping with certain missions. Contacting them in some capacity can also be great story content for Barret since he kind of takes a back seat after leaving Midgar in the original.