I have still been mixing it up, through extensive play ive noticed some huge situational factors with her, While she can be built many ways, I find that The critical Route proves extraordinarily useful in your very early game, my strategy as a solotop is typically do a very early exhaust and burst your opponent, while this may not seem effective, making them burn their pots/ go back early gives fiora all the advantage she needs. Though going with a early BT similar to riven proves useful against a slow low cc team, She imo does not have a set build. Everything and i do mean everything is situational, obviously you will have your core BT and boots, but ive had some tough games where 1 single item chances your threat level entirely. So tbh i dont know every item for every situation, the only way is to test extensively. While the crit runes prove usefull to a squishier opponent, and sustain to keep them down hard, i find straight AD or Armor pen to make her last hits kill at half health at lvls < 6 Anyway thx for the comment, hope u enjoy fiora as much as i do. She is now my Main, and carried several higher elo matches. She is capable of taking nearly any solo top it just depends on your skill of fiora and knowledge of your opponent.
Another note is, Good Complimentary Champions are fairly obvious: Galio ( who can even make use of a tiamat., Zilean, nunu, cassiopiea, shen, amumu, jarvan, nautilus.

BT should be the first item you get before anything else (after boots). I find it extremely useful.. try it see what you think.