Final Fantasy X/Inside Sin

If you're either not interested in, or have already done, most of the sidequests that Final Fantasy 10 has to offer, then this will be the final stop for you. If you've not done most of the smaller sidequests, before you make any progress through Sin you should go and do them; the fiends inside Sin are quite difficult in their own right and the bosses are even harder. However, it is a good idea that if you are intending to get a full experience out of this game and challenge the likes of the Monster Arena and the Dark Aeons, then take the liberty of completing this game before you get too powerful; otherwise the bosses become far too easy and it makes the ending seem a little anticlimatic.