A wise man once said: "He who believeth Final Fantasy XII to be the worst game haveth obviously not playeth Final Fantasy: Crystal Chronicles."
Let's see... the worst game I ever played...hmmm... I'd have to say Xenosaga: Der Wille Zur Macht. Before you flame me, I only say it's the worst game because of how utterly monotonous and boring they made a story as wonderful as Xenosaga play out. It took Namco 3 tries before they FINALLY made good use of their Xenosaga license. By then, it was too late. It was bad enough that they released a (vastly superior) anime adaptation of episode 1 to cover up for the game's dull performance.
btw, if you haven't seen the new dubbings for Xenosaga: The Animation and are a Xenosaga/Xenogears fan, I highly recommend it. I spoke with Chris Ayres (the voice of Jr/Rubedo) about it at Ohayocon; and he said he wished that there had been anime adaptations made for each episode including 4-6.