5th Gen N or Ghetsis

[Amore]

Definitely Ghetsis. With N I usually dick about trying to get the special animations for Fusion Flare (if you're hit with Fusion Bolt or Bolt Strike first the animation changes and looks way more dramatic - kinda reminds me of the end of Doctor Strangelove haha). This tends to make the battle harder than it would be because Reshiram ends up dead / out of pp, but still way easier than Ghetsis. I guess that's partially because my pokémon will usually all have higher levels than Reshiram / Zekrom (which is N' strongest pokémon and my weakest) but lower than that of Ghetsis' strongest.

 

[tamanaphy]

Characterwise, I preferred N, I just liked his personality (if that's even the right word.)
But, when it came to battling, Ghetsis was more of a challenge, which I really liked. I mean, don't get me wrong, N wasn't weak, but he wasn't as strong or as much of a challenge as Ghetisis.

 

[LilyGardy]

Ghetsis is the much tougher battle.

Outside of his Legendary N's team is a joke. Klinklang? Zoroark? Vanilluxe? The Fossils? Seriously and why does two thirds of his team collapse to a Fighting move. Did he not find any Solosis or Gothita whilst on his travels to help cover such a glaring weakness. My Mineshao had a cake walk here.

Ghetsis was fittingly harder. Whilst he also has a bit of an issue with Fighting weakness his Hydreigon is very strong and his Bouffalent and Seismatoad pack a punch too.

 

[Rivvon]

It's kind of a good thing people are saying Ghetsis was the tougher opponent.

Spoiler:

He's the final boss of the game.

I found Ghetsis to be more difficult, too. But they were both fantastic battles.