4th Prince, the chemistry was on fire. But totally agree, he was a bit toxic.
Plus, come on! 8th's prince wife freaking died, he didn't even love her, does he even know how to love Hae Soo? He always stood by and let her go through pain.
Throwing another contender:
14th Prince. Like come on, that boy was loyal, sweet, and took care of her. He even got to marry her in the end