Have I voted in this yet? I really don't think so. If I have, I apologize, and please do discount whatever previous vote I may have had.
Anyway, +1 for The Legend of Zelda: A Link to the Past.
To make things interesting, I think I should give an explanation for my vote. First, this isn't a "nostalgia vote." I didn't play this game for the first time until two or three years ago, and didn't even like it at first. It wasn't until me fourth (or so) attempt late last year that I got hooked on it.
Initially I didn't like it because I was used to 3D Zelda (Ocarina of Time was my first Zelda game, and I had only played and beat the 3D Zeldas since then), but I bucked up and got over the increased difficulty and "outdated looking" style and seriously started to enjoy it. I got my ass kicked HARD, and I think that's part of the reason I respect it so much (especially with modern Zeldas going all, "Master, here, look at this puzzle. Allow me to give you a bunch of really obvious hints on how to solve it, before you even get a chance to figure it out yourself!").
Plus the design of the overworld and puzzles and dungeons, dare I say, are even more complex and impressive than Ocarina of Time and any other Zelda games. Plus, I find it more addicting than any other game. I've beaten it twice this year, and I've been itching to beat it a third, but am forcing myself to wait until early November so I can have a playthrough fresh in my mind when I pick up Zelda: aLBW.
In the end, it's not my favorite game of all time, or even my favorite Zelda, believe it or not. Both of those titles go to Majora's Mask. But from an objective standpoint, I think A Link to the Past is the best Zelda game, and definitely at least a contender for the best game of all time.