Well, it kinda depends... if you're going for accuracy, FF1 is technically "easiest"... but you said you weren't interested in anything before FF7, so that eliminates that, anyhow.
If you're going for "easiest to make recognizable costumes", FF8 hands down. You may not be able to get perfect details, but you could assemble obviously recognizable costumes using everyday clothing, especially from thrift stores and second-hand stuff.
Because of the huge variety, FFT would be easy if you pick easy ones.... or hard if you don't. :P
If you want to get better detail, FF7 or FF9 would probably not be too bad.
And the outfits in FF10 are.... uhhh.... no. Just "no". Don't even try unless you're insane.
, judging from what you've said so far, I highly recomend FF8.