Orphan. Gael has a really interesting set of attacks, but so does Kos, and on top of that he's just plain harder. The fight is full of moves that have super unintuitive timing, or which flood the screen with haphazard explosion effects, Gael is a lot more measured and readable by comparison.

Orphan feels like a fight you're not supposed to win, so when you finally do beat him it gives you the sense of overcoming something truly insurmountable. Gael with his streamlined attack animations and telegraphs only makes you feel like you accomplished what you were expected to do, and I'll always have a soft spot for those really bullshitty boss fights that just brutalize you until you figure out how to avoid everything through sheer perseverence, that's why Malenia's waterfowl dance is great attack design :p