Zuko needs a partner who will be bluntly honest with him and not mess around with the subtleties that he tends to not understand (Toph understands such things, she just doesn't have any patience for them). He also needs someone willing to call him out when he's being an idiot.
Toph needs a partner she can genuinely respect and see as an equal, rather than above or below her.
They both need someone stubborn, but not proud to a fault (ie, willing to stand their ground but also open to compromise).
They're both hard workers who put their all into everything they do (loving, fighting, and everything in between).
They just generally complement each other really well-- both of them have strengths that make up for the other's weaknesses.
Their interactions in canon are great (Toph is willing to trust him before anyone else, and Zuko is more comfortable being vulnerable with her than nearly anyone else, despite the fact that they both barely know each other).
The many narrative parallels
I've discussed this with @authorjoydragon before, and I will make a full post about it at some point, but from a politcal standpoint, Zuko's best marriage option is Toph (hence the appeal of arranged marriage Toko).
Zuko's self-consciousness about his scar, and Toph's total lack of care for physical appearance.
Their similar backgrounds would make it easier for them to understand each other.
