Looking through the Knights I have who are in prime condition for RG (level 5 skill and all equipment slots unlocked for the lowest possible cost), Snake Gourd looks to be decent at burst damage in the first round (2x skill proc rate and guaranteed double strike for self for the first round, +35% attack for self and another 35% for self against bosses, +30% attack for the squad) with a single-target life leech skill.

5* Yarrow is also worth a look with squad-wide buffs of 1.2x skill proc (based on affection), +30% attack, +20% attack in boss fights, and +10% damage dealt in boss fights. Yarrow also has self-buffs of 50% re-act and 30% pursuit so she fits well with someone like Silk Tree who can increase that re-act rate.