Well, I voted for Rikku, Serah and Tifa.
Rikku (FFX) might not be too hard.. I'm thinking of her outfit off of the top of my head so I could be wrong. But from my memory it shouldn't be as hard as compared to Vanille or perhaps Lightning and Yuna.
Serah looks fairly easy as well. I was even considering doing her to take a break from details (like I'm working on right now with Vanille haha). She has a few small details, like the disk-y things on her stockings, necklace and lace on the skirt.. She'd probably be easier than Rikku. Still going off the top of my head here, haha.
And like some of the others I also vote for Tifa. Like the others said, she'd probably be a good beginner costume that shouldn't be too hard. Also, her Advent Children outfit was my first cosplay (it wasn't horribly horrible, but it could've been better - better details and fabrics) but it was also pretty easy.
And just for an extra two cents, Vanille is cute but has a lot of pieces/details to her costume. I don't know how difficult she was for other Vanille cosplayers, but I have tons of materials for her right now and still need to buy some more. All the beading would probably be a pain unless you really like beading. Anyway, I still vote for either Rikku, Serah & Tifa.
Hint.. I would love to see a Serah cosplay (haven't seen any yet!), but it's your decision. This is just my opinion.
ᶜᵒˢᵖˡᵃʸ ᵖʳᵒᵍʳᵉˢˢ ᵇˡᵒᵍ:: (soon)