Answer: Lydia- to Mr. Wickham
Lydia elopes with Mr. Wickham, and after a long, hard search, they are found by Mr. Darcy. Mr. Darcy forces Wickham to marry Lydia at once. He pays for the wedding, Wickham's commission, his debts, just about everything. Later in the movie Mr. Darcy confesses that he did it all for Elizabeth. How sweet!
Charlotte is the first person to be married in the film, however she is not a Bennet. Mary is never married, or even made an offer; poor Mary.