All the FF games are great, but the series is pretty old, so the older games will be, well, more old school.

If you liked the linearity of this game I recommend Final Fantasy X. I think they are similar in terms of pacing. The voice acting is not nearly as good as XIII though. If you liked the battle system I recommend Final Fantasy X-2, which XIII's battles remind me of. Of course, that is a direct sequel to FFX so you might as well play FFX first.

If you just want to play the 'best' FF games then go with VI or VII. Many consider those to be the best. Some hate one and love the other but I love both.