As a F2P player I am very defensive when crafting legendaries, but I crafted Twin Emperor Vek'lor just before Mean Streets of Gadgetzan. C'thun Warrior was a real deck back then and I figured that even after the next rotation it wouldn't lose much so it looked a very safe craft. If only I'd known that Jades would completely destroy any control deck....
I only craft good legendaries, because I do my research before crafting anything. Kind of like how I only watch movies certified as fresh by rotten tomatoes. It's a good way to know what's good... and to save yourself the lost time. That way, when something bad comes along, you can tell. Immediately.
Open the Waygate: I kinda liked the exodia mage when I watched it on stream but I found it very boring to play myself.
Ragnaros, Lightlord: Watched a streamer playing it back in the days, having decent success with a fringe deck he fitted in. Never really played the deck because I was probably too bad at playing it. I like the card itself though, great persiflage of Ragnaros.
My weakest Legendary craft was my second one: Kalimos, Primal Lord. Elementals had just been introduced, and I was relatively new to the game and too hasty in my decision. Elemental Shamans did not last past Journey to Un'goro, although I did get a decent amount of use out of him, and I am delighted to be to use him again in Even Shaman.
Recently, Skull of the Man'ari, which I crafted shortly after the release of The Witchwood for Cubelock, has become a contender for worst craft, since the deck type was nerfed a few weeks afterwards. I made the craft knowing that Cubelocks wouldn't be around forever, and I thought the card had a good chance of appearing in other decks, so I may use it again.