Let's see...

FFI- Never, only played it once.
FFII- Haven't even finished my first run through.
FFIII- About 3 or 4 times.
FFIV- Once, haven't finished replaying it though.
FFV- Not yet, but I was planning to soon.
FFVI- On my second or third right now.
FFVIII- Still on my first play.
FFIX- Same as above.
FFX- Once.
FFXII- Haven't finished my first go yet.

I really like replaying certain FFs, as you can see.