Anna already has a guy.

Anna probably married Kristoff soon after Frozen. Elsa is 21 so eventually she would find a good suitor. Meirda is 18 and she'll probably wait many more years to find somebody. She said herself she wasn't ready and would probably never be ready. Though she could marry if she fell in love with someone.

^

Hopefully Merida finds a strong, sexy, kind man.

My baby. It's okay Merida, take your time. And don't come back with a man until he's, rich, handsome and let's you ride with your hair flowing in the wind.

Anna already has Kristoff.

It may take awhile but she'll have to marry. Especially since the peace of the kingdom rests on intermarriage (although she may find a way around that :)

Anna has love interest.
I think Merida would end up with some kind of royalty guy :)

Anna already has Kristoff.
Anyway, I'm voting Merida for this one. We all know how much she wanted her freedom that she was willing to change her mother for it. And how hesitant she was at the idea of marriage. Elsa is 21, I think she'll find a good suitor in time.

Anna has Kristoff, so she's most likely to get married first.
As manu and emerald said, Elsa is 21 so she'll probably find a good suitor soon.
Merida made it clear that she doesn't want to get married. She'll probably find true love soon, but I don't think it will be anytime soon.