Since you've already played some of Clover's other titles, not to mention Devil May Cry, you may as well go for Okami. I rarely enjoy long games. I'm 65 hours in and counting and I'm still intrigued.

Ico and Shadow of the Colossus are reason alone to own a PS2. If you haven't finished both of them, you need to get on that immediately.

KojiPro's Zone of the Enders: The 2nd Runner is my absolute favourite action game of all time, and my favourite game to come out of the studio. If you enjoy Metal Gear and haven't played MGS2, you should at least try it. The story is generally love it or hate it, but the gameplay is absolutely AMAZING regardless, and even better in the Substance director's cut (VR Missions w00t!).

If you've played Soul Reaver 2 and enjoyed its storyline, Defiance is a must. The game is a great finale, especially when you consider how much was against it: a severely limited budget, marketing pressure, tight time limitations, and the team of writers being forced to accept Blood Omen 2's storyline as canon and work around its utter ridiculousness. Defiance is a bit rough around the edges because of these things, but ultimately its acting, writing, and general storytelling panache shine through in ways most other games can only dream of.

Rez. The ultimate rail-shooter. It's more comfortable in the format than nearly any other I've played, and it blends play with sheer artisitc WOW better than any other title I've come across.

Shin Megami Tensei: Nocturne is one of the greatest RPGs ever made. Its storytelling is a godsend in a staling genre, its play is challenging and strategic, yet unrelentingly fast-paced. The world is like an interactive Dante's Inferno, and every single aspect of the game comes together in astounding ways. It is the ultimate game in the series, as far as I'm concerned. A perfect melding and streamlining of all that had come before it, and none of the games made since have had quite the same impact.

I really love both Digital Devil Saga games as well. The gameplay is basically watered-down Nocturne, which makes it a solid starting point. The story is great, but more traditionally told, making it a bit more accessible as an introduction to the series. I haven't played much of Persona 3, so I can't comment. I can say that in spite of lacking both the series director AND lead artist, everything seemed like it came together pretty well.

I can't recall now if they were on your list, so if not: Silent Hill 2, Fatal Frame II: Crimson Butterfly, and Tales of the Abyss are games you can't go wrong with.