fioras ult

You shouldn't be participating in teamfights at all with fiora. She excels at splitpushing and duels, or 1v2/1v3s if you're good and fed enough. In which case, you ult the carry and burst them down before they kill you and deal with the less damaging tank afterwards.

It doesn't matter who you ult, so ult the one you can reach the easiest and need dead the most. And dont forget the psychological warfare;

If I'm playing Twitch and an 8 CS/min 9/1/4 Fiora Qs in my direction and then presses R and my team cant instantly kill her/she riposte's the CC, I'm stealthing and running away before she ever gets close. In other words, if you have an advantage, just using your Ult at all forces the target to leave the fight because they cannot let you proc those vitals. As long as youre building some level of AD and can get your E off, already gets your R to "guaranteed death" status on top of healing you for several times your max HP

just ult whoever is easiest to takedown and get that heal off

Each Vital does %hp true damage so ulting a tank isn't bad.

if you must teamfight with fiora then you are the mehanical anti-carry. you apply your ult to whoever it needs to be applied to in order to win the teamfight. get it? got it? good.