Who should you focus in a fight?

I don't know about other people but for me, being an assassin player, i focus the back line and anyone that i see from vision who's alone.

Basically if it's squishy, i want it's tushy.

But in a teamfight who should everyone else focus? Like, 90% of the time when a fight breaks out, it's a 4v5 because i killed the mage or the ADC, or the support, or both so it's a 3v5 but like, it never turns out that way. What usually happens is my team focuses the Udyr, Gnar, Ornn, Braum or any other tank and just gets melted by whatever other carries they have and this frustrates me to no end because i'm not good in teamfights, instead i like to make teamfights 4v5's or 3v5s so my team can easily take care of it but i feel like a lot of people don't know what this is.

"Order of Threat" basically, in a fight you should focus the people with the most threat level in order, now this changes a lot if you have a Heal Bot Sup or something but most of the time it goes like this, atleast to me.

ADC-MAGE-FIGHTER-SUP-TANK

Sometimes like this

MAGE-ADC-FIGHTER/JUGGERNAUT-SUP-TANK

And Very Infrequently like this

FIGHTER-ADC-MAGE-SUP-TANK

Lastly as i said above it can go like this.

SUP-ADC-MAGE-TANK

Like, do i have something wrong, is there an instant in which focusing the tank is the best option? I understand focusing fighter's or diver's down but full tanks? I just don't get it, all your doing is wasting your ammunition so the damage dealer's can wreck you.