That's a difficult question to answer unless you're pointing to a specific kind of remake. Which naturally means I'll present all of my possible opinions, so bear with me.
In terms of graphics only - Oracle of Seasons. I haven't actually played the game, but imagine the game in stereoscopic 3D on the 3DS or HD on the Wii U. Each place would look different depending on what season you'd changed it to, right? With colorful, detailed graphics, it should look beautiful, especially if it was remade in 3D like OoT or MM, but that is merely conjecture.
In terms of merely bringing an older game up to speed - Probably the first Legend of Zelda. It held some brilliant concepts that were unfortunately pushed into a corner as the series developed, and utilizing these concepts (open world exploration, tons of secrets) in modern Zelda would be amazing. Regardless of whether it's 2D or 3D, the first Zelda would benefit tremendously from things like sidequests, more mini-games besides the "money-making game," and perhaps a dungeon creation system so you could share content with others. The game could be incredibly intuitive without even trying hard, I know it.
In terms of a massive overhaul - Link's Awakening DX. "Massive overhaul" means new graphics, new story execution, new sidequests, etc.. Starting from the beginning in a bigger move than Ocarina of Time 3D. Link's Awakening was a very personal story, or at least it would have been more so to me if it didn't seem so scarce on story in the first place. It also seemed a bit dry of sidequests and mini-games, making it ostensibly one of the most linear-feeling Zelda games in the series, right up there with Skyward Sword. Modern storytelling techniques, new dialogue, a somewhat redesigned overworld to make it feel more like an open island, sidequests, and mini-games could turn Link's Awakening into the masterpiece it was always meant to be. I like it as it is, but there's so much more for it to realize that a complete overhaul remake would turn it into gold. It's more for the story than anything. I wanted to feel more for the ending of the game, but the speed and detached attitude with which it presented the plot kept me from that.