Well, it's got to be IX for me because I don't recall any tense moments in it. FF7 had that train that you had to get off of within a time limit or it'd be game over. I never failed it, but it's definitely not relaxing! FF8 was worse with the spider thing chasing you, and you were on a time limit! And ugh, the non-stop enemies after the Yuna/Seymor wedding in FFX was nerve wracking cause I was afraid I would die and I knew it had been awhile since I had seen a save point. Final Fantasy VI had the timer at the end of the floating continent... I remember the first time I played through it I missed Shadow because I got off as soon as I could because I was pressured by the timer. Final Fantasy I-V weren't easy enough to be relaxing (unless you count Dawn of Souls...)

So yeah, basically, if you put me on a timer I can't relax. lol