This is a very tricky question to answer, because all children are different. No one size fits all, so what is right for one child is not necessarily right for another. Maybe your eldest child is very sensitive and your youngest is very tough, so how do you approach them when you need to reprimand them?
Maybe you have to adopt a softer line with the more sensitive child, and be more severe with your youngest, but then you can be accused of favouring your oldest. But of course you are not. It is completely normal that you might empathise more with one of your children, particularly if he has a complimentary character to yours, but this is not to be confused with favouritism. You are managing the situation with different strategies for each child.
So long as you avoid giving things to one child and not to the other, then you are certainly treating them all equally. It is of course possible to give something to one of them and something different to the others, but make sure they all get something which is interpreted as fair for everyone. If for example you take one child on vacation with you, and leave the other at home with grandparents, then this is definitely not fair and would certainly be showing favouritism towards one over the other. The problem would then be that you create jealousy between your children, and resentment towards each other. If you can’t take them all, then you shouldn’t take any of them.
Favouritism is to be avoided at all costs, as if you want your children to grow up and get on with each other when they are adults, then you have to be careful not to create resentment and jealousy between them when they are youngsters.