As far as the Istanbul teams, we all hate each other. Of course fener comes first by far. The only difference is, each and every part of the country hates fener. Because they're always involved in match-fixing but never charged for what they've done. Platini always mentions zero tolerance for match-fixing but he doesn't do shit about fener.
Galatasaray on the other hand has done some amazing things in Europe since 1950s coming from a country like Turkey against big teams financially 100x better and stronger. So people outside of Istanbul looks at this as a national success and supports Galatasaray in the European matches unlike fener who is all about themselves.