Fiora is kinda trash teir, if you get fed shes fine but if not you're just going to be nothing.
fiora gets countered by tanky top-laners (like malphite, volibear...), or by bruisers that do good damage (like darius, yorick...), or in short by most top-laners.
you dont get realy tanky, and you cant do realy good damage under cc. your only good damage output in team fights is your ulti, and you cant control 3/5 of who it damages.
i bought fiora and had some awesome games. but she gets owned and becomes boring quick.
so you can buy her, but know that most of the time you will have better champions to pick instead.
with that said, other champions i recommend to you for top lane are: darius, singed, voli, malph, riven. all OP, and all have more good use than fiora.
I play Fiora from summoner lv. 5 to 30, and, yes, the above comment is the truth. She is just for entertainment, honestly. Like, if you are able to time super correctly, you can clean up in teamfights by ult (sometimes Q + ult). If you wanna win desperately, don't buy her. She has no poking skill and her Riposte is nerfed. Riot made her can only parry autoattacks, not those skills like Pantheon's spear shot or Mundo's Cleaver, etc. In all, playing her requires extra-ordinary skill in timing and using speedy combo, but having all those skills does not warrantee that you can beat a noob with a better champ. That is why I play Pantheon now. He's cheaper, more victorious, and not less entertaining than Fiora.
A truley skilled fiora can basically all top laners except for jax. His kit just counters hers to hard. Her late game is insane, considering how shes a carry champ and it should be. Her lack of cc is unfortunate but when underfed she can still become a excellent peeler and still be able to demolish an enemy AD carrey. (This is with frozen mallet of course, which i find to be a pretty required item on her) I have played over 600 games with her over 2500 top games in general (im a top only player). Do not let anyone discourage you from playing her, shes an excellant solo queue champion because a fed fiora is pretty gg and its easy for her to get fed since many players do not have much experience playing against her. A good Fiora can easily beat Singed, Volibear, Riven, (espically riven) Elise, Pantheon, And even Malphite to an extent but it requires mastery, despite what people say shes a difficult champ to play correctly. I hope that helps clarify Fiora a little better than the above.
I have played Fiora at top and while it is not the easiest thing to do it can be done. One of the biggest suggestions I make is roam to mid lane when able too. This will help not only you for the assist but also help your mid out a lot. Fiora also takes time and a lot of skill to perfect. I am not a perfect or amazing Fiora but you just have to play your match ups.
Fiora has been buffed as hell with the new BotRK. Its active is great for duelling, and the proc can wreck an hp-stacking bruiser within seconds with the Burst of Speed.
Fiora is a really strong champion, providing you know how to play her, and yes, like all champions she has her "Counters" however as with all champions there are no "100% win matchups". All the "counters" to Fiora have their weaknesses, Pantheon's is being mana hungry in the early game, Singed and Malphite are extremely weak early on and both (especially Malphite) are mana hungry. Also, the best way to play Fiora is to max lunge first, this allows for much better harass and burst. Also, to clear a common misconception Jax does NOT counter Fiora, in fact it could be said that Fiora counters Jax since both their kits counter eachothers, both have a dodge spell and both are AA reliant, also Jax has a sh*t early game and Fiora Has a strong early game. And with the new Blade of the Ruined King she is really strong and can shread tanks and brusiers easily, plus is makes up for her lack of CC.
Fiora is a strong champion for sure, especially in late game, with life steal her ult can get her back from 0 health to full health. To me, she is one of the most OP champion.
I know this is an old topic, but what kind of weed were you smoking when you said that Fiora counters Jax?
Jax is the best Fiora counter. Period. He can outdamage her in any time of the laning phase, even in the early game and survive her "burst". Fiora has her w which blocks one autoattack for some shitty damage in return, while Jax has an e which blocks/reduces all of Fiora's damage. He also simply has more damage and is tanky enough, even in the early game to survive her so called burst.
Malphite > Jax against fiora every time.