Most Disappointing: Probably a two way tie between Final Fantasy Tactics Advance 2 and Banjo Tooie.

Worst: Super Mario Bros The Lost Levels

Usually if I don't like a game I won't finish it. Currently the most disappointing game that I can think of is Super Monkey Ball Banana Blitz. The two Gamecube games were amazing, but the Wii titles just... sucked.

Most disappointing : Battlefield 3, any version (PC, consoles), doesn't matter. Or maybe FF13, just too hard of a decision.
Worst : A cliche answer but ET.

Undecided, but heavily lending towards no. It's going to have to get overwhelmingly positive reviews before I consider going back. 

If you do decide to try it out, and you want a group/clan to join, let me know. =) On PC of course.

Most disappointing: The Legend of Zelda: Skyward Sword

Worst: The DS version of Need for Speed Underground 2


Yup, high standards over here.

Edit: Whoops, forgot to tell why.

Skyward Sword - Slow start, too linear, no real overworld and thus not much, if any, exploration. All this despite an extremely long development time.

NFSU2: Really only had one mode (as far as I can remember) which was career. And that mode wasn't rewarding in any sort of way. Thanks to Mario Kart and the far superior sequel NFS Most Wanted, this title became what I believe is my least played game in history.

the worst game ive ever played is Power Rangers: Lightspeed rescue on the N64

probably the worst 3D game ever created, absolutely pure garbage on every single aspect,
horrid , clunky , boring, senseless.....gameplay mechanics, awful grafix and zero lasting appeal

yes, its worst than Superman 64

I think more people should explain why, and to liven things up, perhaps try to not keep it to games that are notoriously bad, like Duke Nukem forever or ET or Superman 64.

Magna Carta for PS2
I wanted to love this game so much, the art style suckered me into a purchase immediately. I start playing and realize the game makes me sick to my stomach. I toughed it out for 3 days before calling quits and took it back.
-Main character, Calintz. Totally thought it was a girl....well its not. It probably WAS a girl at some point during dev but then they decided US/EU territories might not take kindly to a fo-lesbian love story and they simply swapped voice actors and changed a few lines of dialog. Unfortunately they never touched the actual design and HE very visibly has breasts in the artwork and in-game. Also has an overbearingly macho voice which doesn't suit him at all.
-Battle system was crazy broken. The Talisman mechanic will totally screw you over very early on.
-cost me like 70 bucks because it came with a really nice art book. Unfortunately the art book was the only thing worth keeping.

tied with....

Dragon Quest VIII on PS2
-I....HATE....1st Person point of view in RPGs. I think it is the most retarded thing in existance. Everytime I go into a random encounter and all I see are 3 enemies and menus..........UUUUUUUUHHHGGG!!!!!
-Not the biggest fan of the DBZ art style. I tolerated it in Chrono Trigger because the game was just that awesome. Yet due to this game have the pet peeve above, no.
-I bought it mainly for the FFXII demo inside it. Played the demo more than I ever did the actual game.

I usually have exceptional taste in video games and have a keen eye to spot only the worth while ones. I had a brain fart on oth of these, both clouded judgement due to pretty add ons.

oh and as to not miss a chance to rag on FF, Final Fantasy XIII is by far the biggest disappointment. I had such high hopes and they screwed me on like every front.

-Long Dev Time
-Was suppose to be the PS3 masterpiece, fully utilizing most of the system's specs and bluray to create the dream RPG. I realize now this was a fantasy within itself.
-Postponed release simply to shoehorn a 360 vers in at the same time as PS3 when PS3 gamers have waited half a decade and wait all the time for exclusivity to end on 360. Beyond infuriating.
-Game comes out and is a total bust. Weakest characters in the series save a few, even worst than FFXII's.
-Can only control one character in battle. Who's awful idea was this?
-No exploration for majority of the game and the parts that are big are fictionally made to feel bigger than they are because they dont give you typical methods of transportation like....idk....an AIRSHIP!!!
-Corridor city...
-Shortest FF I have ever mastered(Platinumed it in 200hrs, every other FF game takes me 300+ to do EVERYTHING in it). Most of that time was spent petty grinding a few super rare items.
-Story by far the weakest in the series. The back-story and mythos is far more interesting.
-Traditional Summoning staples like Ifrit and Shiva are totally butchered and on occasion swapped for god knows why. Spell names are changed for no reason as well instead of using the FF mythos's typical terms.
-Part of the reason that my lover's game(Tetsuya Nomura) has taken an extra long time due to the FF Versus staff helping with this turd's development.

Unfortunately, even with all the things that make me hate this game, it is not the worst by far. It is on the other hand the WORST FF I have ever fully played.