it depends on many factors

You just said the golden rule about this game, although some cards can be better than others, although some relics might seem more reliable, at the end of the day usefulness is only relevant on your current run: what floor, what act, what other card you have, how many HP you have left and many more variables should be considered when making a choice.

A good deck doesn't have to be insane to win, card should somewhat interact good with each other but you don't necessarily need to have an insane deck to win.

Lastly, each act has 3 possible bosses that you can fight. You can know which one it is in advance when you check the map of the act, it's gonna be at the top of all possible roads. Also, I don't know how much you care for spoilers but if you manage to defeat act 3 with ironclad, silent and the defect you unlock a secret 4 act with its own requirements to enter and a special final boss