Gah! You're asking me to pick between five different awesome things. It's like a parent trying to play favorites on their children--well, at least if the children are good
How am I supposed to answer this!
Let's see. Ruin's manipulations made the whole Book Two worthwhile. If there was not that plot twist, I would not have liked Book Two very much. The whole book relied on that plot twist, and it made the entire book satisfying and amazing. It completely surprised me.
Kelsier's plan made his death worthwhile and one of the best character deaths I've seen in a while. Combined with the revelation about Rashek, these two things made Book One amazing.
Sazed being the Hero of Ages. Well, this is fantastic foreshadowing and he healed the world in an extremely satisfying way, which is the biggest plot thread that had to be resolved (bringing the flowers back, essentially). I can't think of anything better foreshadowed than
how Sazed healed the world, because all of those religions had been introduced throughout the series.
...Writing this, I am no closer to deciding which were the best twists. Kelsier's plan, while cool, would be eliminated first in my opinion. That makes it no less awesome, but Rashek/Ruin/HoA? Those are mysteries from the epigraphs, tying each book and the overall mythology together. Kelsier can't compare with that.
So I'm not going to vote and choose between those three. It's too difficult.