Worst bits:
Relentless behaviour when you just NEED it to stop and they don't give a shit (not because they're horrible, but because it's not even remotely on their radar). Whether that's the baby needing you at night when you need to sleep or the toddler moving at a snail's pace when you're late or the preteen who won't stop talking when you need mental space. Anything like that.
Potty training, especially accidents (cleaning them up)
Crowning. Actually most of labour but mainly crowning.
Endless cooking groundhog day, especially spending £££ on food you then just throw away 
The bits you can't get right. I thought I'd be able to read a book or a few websites on parenting and ace it like I'd always been able to do at school, hahahahahaaaa nope.
The weight of the love/responsibility.
Best bits:
Seeing them become their own person, finding out what makes them tick and their passions etc - magic.
Watching them learn things. Just amazing. They are born and they don't even know that they're a human yet or who you are but their in-born curiosity is incredible. And then seeing them pick stuff up over the next few years and the realisation of what a massive task it is to learn to be a functioning human. DS2 has just learned to ask us to do things by bringing us the associated thing e.g. socks when he wants us to get out of bed, shoes if he wants to go outside, a bowl when he wants a snack. He can't talk yet but he's learned that we are interested in communicating with him and he is figuring out ways to do that without using words, how awesome is that??
Seeing what bits of you/DP they "get" (traits, looks, etc) - obv only applicable to bio children so not an essential one but fun.
Hilariousness of 1-5 year olds. Actually babies are funny from about 4 months old but they get utterly hilarious from about 1. They are just nuts, it's great.
The enormous privilege of knowing somebody right from the beginning of their life. And the incredible smallness and briefness of newborns.