Your definition of art is extremely restrictive. The assertion that art can only function on a purely narrative, visual or dialogue based level in regards to games disregards one of the greatest strengths of gaming, interactivity. As mentioned above some of the best games ever created aren't exactly pieces of renaissance art in terms of visuals. I think the fact that games allow an interactive experience (as opposed to the sit back style of other mediums) is the mediums greatest strength.

However in terms of narrative, dialogue, philosophical themes and art style I'm going to recommend Planescape Torment to you. Its a pc game created by Black Isles that is essentially unsurpassed in regards to narrative and dialogue.

I think the fact that video games contain art created by and art team it logically implies that video game are art.

The simplest answer is the best one.

Well, since u already have shadow and Ico, there are few that can stand out like those in the terms ur looking for, but here are some:
Okami(innovative and great quality, what I call and artistic game)
almost6 any FF(great animations, the best music IMO, great storylines and tech advanced)
Folklore(even thought they called it a flop I think it awesome. The music is really good, it has a mysterious aura in it, like joining the popular crime novels with the world of the fairies, in a very good way)

I cant remember more, but u could use god of war since its like an epic movie.

Okami, Okami, Okami, Okami...You don't need more;.

I forgot about Radiata Stories, I liked that one.

It depends on what exactly you are looking for in terms of art. I will break up my recommendations into two general categories, games driven by a compelling story vs. games driven by art direction. I will try not to include games that have atrocious gameplay.

Story driven:

The MGS series (particularly 2 if you are looking for a highly politically charged avant-garde style storytelling, and 3 if you are looking for more traditional commentary about the industrial-military complex and patriotism)
FF VII and FFIX (I think these are the two best representations of unique story and gameplay from the FF series)
Xenogears (an extremely ambitious RPG whose script I have reread by itself because it is so compelling)
Xenosaga series (self-indulgently story driven, but for those who are patient enough with it, it is at the same level as classic literature)
Silent Hill 2 (the best representative of the "horror" genre of video games. A very open-ended story that invites interpretation and leaves the player both compelled by what is going on around him and absolutely frightened at the same time).
Final Fantasy Tactics (extremely motivated by the history of the Catholic Church and European history, this is arguably the best story ever in a game)

Artistically driven:

Okami (great representative of stylized Japanese culture in physical form)
Everyday Shooter (a great downloadable PSN game that is equivalent to a psychedelic experience)
flOw (simply beautiful and elegant in its simplicity, also on the PSN)
Castlevania: Symphony of the Night (a great embodiment of "old-school" gaming that draws great strength from its gothic atmospheres)
Prince of Persia: The Sands of Time (extremely innovative gameplay wise and absolutely gorgeous in every other way. Very compelling representation of Eastern culture, particularly architecture).
Gears of War (it may not have the best script ever, but it has beautiful and cohesive art direction).

I can think of more, but that is a very good list I have given you. Out of all of those if you are truly looking to research games as art, I recommend MGS2, Silent Hill 2, Final Fantasy VII, Xenosaga I-III, Okami, and Everyday Shooter.

Also, saying that interactivity with the user means something is not art is an inherently flawed definition of art. Particularly in the last 50 years, art has opened up to include the "reader" or the "user", however you define it.

Some recent and artistically significant works of art are now interactive in that they allow the "reader" to physically touch them and move them, or even to walk through them. Performance art has opened up to the audience as well by including the audience or allowing the audience to influence the direction of the show as well.

The relevance of art changes from century to century as well, and the role the general public and art critics play in those changes is entirely relevant to not only what the art piece was to begin with, but what it has become. To say that art needs to be or even is immutable in and of itself is a narrow perspective of art.

The problem is, you're trying to base games as art around the concept that what makes them art are other artistic mediums. There's music, which is art. There's still frames which are art. There are moving pictures which are art. There is a story which is art. None of these make games art. They make their original components art. What you need to find is what makes games an artistic medium on it's own merit. In my opinion it's the interactivity that makes it art. Steer clear of music, still frames and moving pictures because all that proves is that the three of them are artistic mediums and gaming is borrowing them.

Gears of War has many images in some big time art books.

Bioshock

Posting in a "games as art" thread.

Simple answer: All games are art; it's just that most aren't particularly GOOD art. That said, I'm a bit of a connoisseur of artsy games. If you've already played the Team ICO games, Rez, and Katamari, I'd check out, in rough order:

1) Planescape: Torment. The closest thing to a Citizen Kane that the video game industry has ever produced. Simply amazing in every way conceivable.

2) Killer7 and No More Heroes, and possibly Contact. Suda51 has a reputation for developing bizarre, post-modern games, and it's a reputation well deserved.

3) Silent Hill 1-3. All three are worth playing, and all of them are disturbing in a David Lynch's Eraserhead sort of way.

4) Shin Megami Tensei: Nocturne. I can't think of very many JRPGs that are worthy of the title "artsy," since the genre is largely crap, but this one definitely is. (As a bonus, it's also one of the best JRPGs ever made.)

5) Any of the Oddworld games, but especially Abe's Odyssee and Exxodus on the PS1. Bizarre puzzle/platformers that are well worth playing.

6) Fear Effect 1 & 2. Sort of Resident Evil crossed with Blade Runner, as odd as it sounds, and one of the few games out there that would probably make a good movie.

EDIT: You may also want to watch out for Eternity's Child, a hand-drawn platformer coming to Nintendo's WiiWare service. Ever since finding out about it and seeing some screenshots, it's been up there on my "most anticipated titles of 2008" list.