For me is right now. DS is now pulling up on anything.and.everything and must have constant spotting in case he falls (which, unfortunately, is still kind of frequent). He has a fit when I put him in his exersaucer to contain him if I need to use the bathroom, get DD a snack, etc. So I basically spend all my time sitting right next to him.
When I put him down for his nap, he spends the next hour pulling himself up and then crying for me to help him down. This then causes his morning nap to be pushed back a little later, which means almost as soon as he gets up, DD goes down, and when she gets up from her one nap, he's ready to go down again. In the past week, I've been tied to the house from 10-5 because someone's always napping!! I can't get anything done outside of the house, and I can't get much cleaning/housework done because they aren't napping at the same time anymore. I know it won't be too long until he's more steady on his feet and things will get easier, but man, this is tough right now!!
Thanks for letting me vent:)
Re: Most challenging part so far...
Oh no! That sucks! I was telling my DH that I think we're going to have a lot more issues when DS2 starts becoming mobile.
Hang in there!