Col Fitzwilliam is the obvious choice and then Georgiana but I enjoy reading stories about the other characters, particularly if there is still a little if D&E included
To be honest,it depends on what’s happening in the story. Col. Fitz. can be Darcy’s very best friend,realising he’s fallen for Elizabeth and endeavouring to help in any way he can to see them joined in matrimony.
Other times,if he’s fallen for Elizabeth first,there can be tension. However,he normally holds a special place in my heart.
Darcy’s cousin, Colonel Fitzwilliam, seems to be one of the only characters that understands him. I usually dislike stories where he betrays Darcy, because it just seems to be contrary to their relationship.