I've played:

FFI (Completed on NES, WSC and PSX, also played on GBA)
FFII (Completed on NES and GBA, also played on WSC and PSX)
FFIII (Completed on NES)
FFIV (Completed the FF"2" SNES, FFIVj Hard Type, also played on WSC and PSX, currently playing the GBA version)
FFV (Completed on SNES, also played on PSX)
FFVI (Completed on SNES, also played on PSX)
FFVII (Completed on PC and PSX)
FFVIII (Completed on PC, also played on PSX)
FFIX (Completed on PSX)
FFX (Completed on PS2, the PAL version)
FFX-2 (Completed on PS2)
FFT (Completed on PSX)
FFTA (Played a few hours)
FFMQ (See above)

The next FFs I'm thinking of playing are FFIII DS and FFXII, although they'll probably take quite a long time to get released in Europe.