Definitely Zelda. No doubt about it. Okami was a good game, but I didn't enjoy it nearly as much as pretty much any Zelda game I've played.
Graphics wise, I'm not really sure which is better. They have completely different styles: while Okami is beautiful, and does capture the atmosphere perfectly, I prefer Zelda's variety of different, but very effective, graphical styles. Probably I'm just biased, but there you are.
Story wise, Okami had a much more detailed presentation than most Zelda games. Even so, I just didn't care about it all that much, whereas the more in-depth stories in the Zelda series always had me wanting to find out more history, or wondering just exactly what is going to happen in the end (beyond Ganon dying, which is pretty much a staple
).
Gameplay wise, Zelda wins outright. It's got awesome puzzles, fun combat (and great boss battles), really cool items, awesome dungeons, fun and rewarding sidequests, and so on. Okami has the "items" in the form of the brush techniques, and some of them were definitely really cool. However, everywhere else it was kind of lacking in my opinion: most of the puzzles were just not as good, the combat was pretty tiresome (even the boss battles felt sort of tedious to me), the dungeons, while interesting, weren't all that great, and the sidequests, while numerous, weren't very fun
or rewarding.
Music wise, they're actually pretty close. They both have pretty epic soundtracks, and the music always fits the mood for both of them. Probably I'm biased once again, just because I've played it a lot more and have lots of nostalgia for the tunes in it, but I really do like Zelda music more.
Replay value wise, Zelda wins once again. I can usually replay a new Zelda game once right away, even if I've just beaten it, and then every year or so I feel like replaying several of them, and they're still fun. However, with Okami, I have no urge to play it again at all, even though there are a lot of things I haven't done yet. I just wouldn't be able to stand going through all of the parts I didn't like, only to have a little bit of fun playing the parts I did like over again.
So Zelda wins in every category, for me. Okami was good: I mean, it practically borrowed most of it's gameplay elements from Zelda, and Zelda is awesome. However, it just didn't get any of them right quite like the Zelda series does. I suppose that might be because Zelda has had a whole series for Nintendo to get things right, whereas Okami has only two games in it, both pretty recent. Maybe if they keep going, and fix some things up, I'll really start enjoying them more.