Hmm, you should've included the CD-i games. Even though they weren't made by Nintendo, they officially licensed Phillips to make them, so they can be considered official games and make it into the list (after all the Capcom games are there too).
Even though I haven't played them myself I've watched enough gameplay videos (including the whole ZD video walkthrough!) and the totally broken gameplay mechanics, pathetic bosses and cheesy cutscenes really make these the worst games that officially bear the title "Zelda".
If I consider only the games I've played (see my signature), well then "worst" sounds a bit harsh because there really isn't a bad game in the list.
But if I have to pick the one out of these which is my least favourite then it's probably WW which I'd still give a 7/10 rating, meaning it's still a good game overall. The main issue for me weren't the graphics, I'm fine with the cel-shading in general but I would've preferred less extreme body proportions (maybe more American cartoon-like) and maybe just a bit less contrast and brightness but like I said this isn't the biggest problem.
The things I didn't like particularly are:
1. Tediousness. Getting from A to B took ages because you couldn't speed up your boat (why was there no sail upgrade(s)?). Sure, the Ballad of the Gales did help somewhat but the places you could warp to were few and far between, why wouldn't the game let you warp to every single square? Oh, and don't get me started on the Triforce chart/piece quest which was the mother of tediousness. The problem could've been easily solved or at least encountered a bit by condensing the overworld to 1/2 or maybe even 1/3 in both axes, which also would've been a solution to the next point...
2. Empty overworld. There was just too little going on on the Great Sea, islands were small and the vast majority of them only had a single purpose and were only visited once, sometimes just for a heart piece or some rupees.
3. Too low difficulty. I haven't seen the game over screen once in this game and I don't remember having had to use a fairy. Most enemies did only 1/4 heart damage and most bosses usually 1/2. Special attacks that are more dangerous were rarely used and easy to evade. Only Ganondorf could deduct a full heart at once easily, IMO all bosses should've been that hard. And of course we had that somewhat boring "defeat dungeon boss by using only the dungeon item on an obvious weak spot (usually red flashing eyeball thing) and then slash with sword" scheme again.
So after all those flaws you might wonder why I'm still giving the game a 7/10? Well I think we're sometimes complaining at a high level when it comes to Zelda and take many of the positive aspects as granted.
For example the controls were very intuitive and responsive, nice extra attacks like the "backslash" and whirlwind were introduced, dungeons were themed well and some of the puzzles, although mostly easy, were fun. We also saw a lot of new and innovative items which other than in the case of TP were put to use often and also a lot outside of the dungeons they were found in. And finally the game had an intriguing storyline with lots of silly and funny moments (especially with Tetra).
Therefore I'm not giving a vote right now because it wouldn't be fair, also considering that I haven't played the DS games yet and from what I've heard they're not really in the top lists of most Zelda fans.
Don't get me wrong, I love this as a DS game, but it fell short when it comes to Zelda standards. 
