Of course by HD I mean something like todays gaming PC, and possibly Nintendo's next system depending on the direction they take.

Pretty simple question, what Nintendo games would actually benefit from a visual upgrade?

In my opinion most games from Nintendo would gain nothing going to HD like Mario. But I do think a Metroid Prime game would gain a lot with a visual upgrade since the series is built around very detailed, intricate environments. With a more powerful system I can't even imagine what they could come up with. Since there are rumors that the Metroid Prime series isn't over yet, it has me even more excited.

I love the Wii, but I'm still extremely excited for Nintendo's HD system.

Metroid Prime for sure, but The Legend of Zelda could really benefit from it as well.

Well nintendo decides a concept first then they worry about the details later such as the graphics so it really wouldnt do anything for current franchises, but it just opens up more options for later.

Metroid, Zelda, Pikmin, Starfox and Pilotwings

From my experience with HD, the only things which see a substantial benefit from it are nature documentaries, some sports, and things with lots of text. I just watched Star Trek on Blu-ray and I'd be just as happy with a DVD and ten more bucks in my pocket.

So I'd say Animal Crossing, due to all the letters. I really can't think of anything else offhand.

I don't know about LoZ. Windwaker was incredible, and wouldn't benefit from HD

If we're allowed to venture into the added hardware that benefits the HD titles past merely the visuals, I'd say a sequel to Odama would benefit the most. Allowing more intricate AI patterns to smash with your pinball could do nothing to hurt the game and only add to it. I suppose in a similiar way, it could get more Pikmin on screen at once also.

This is a thoughtful and well-thought-out answer, but the OP is asking from a purely visual perspective. That may still apply to Pikmin, though.

I don't know if I can thinkn of any that would actually really benefit in substantial ways from a visual upgrade. I mean of course it would be nice and I'd scream and jump up and down and everything if the new Zelda pushed technology in whole new ways, but...

Metroid Prime, F-Zero and possibly Star Fox, I'd say.

Nintendo doesn't make games that benefit from visual upgrades, outside of the obviousness of the upgraded visuals. There is no benefit other than it simply looking better. The only major changes Nintendo series have made, were not because of the visual upgrades allowed by new hardware, but by the power they had behind them to allow new experiences. For example, the transitions of Mario, Zelda and Metroid into 3D. Everything that comes along that gives each generation of games it's added dimension doesn't come from the visual aspect, but from what the new consoles allow you to do with the added power underneath the hood of the console.