This gem was made made Squaresoft back before they merged with Enix Co. and became Square-Enix. First off, this game had one of the most exciting soundtracks of all time. Right off the bat you're thrown into the action with this track here:
From there it just gets better and better as you adventure through treacherous territories and overcome outrageous bosses. If you consider games like Spyro and Crash platformers, then this was easily the best Action/Adventure game on the PS1. The awesome gameplay and story is only added to, in my opinion, by hilariously cheesy dialogue, as well as a story that's not half bad.
Also, there was a Simon-esque minigame at the end that was pretty fun too.
Anyway, sorry for the abundance of video links, but this game just has too many good moments - making it #5 on Content Hedgehog's Top 5 Video Games of All Time.