I've been creating since LBP1 but I don't remember it being 'hard' because I was younger. I personally prefer LBP2 and LBP3.
But for someone interested in creating who hasn't played a LBP game or hasn't used its create mode yet, which mainline game should they start with?
LBP2.....that's it.
I think it's probably a good idea to start with LBP1 or LBP2. LBP1 you get this very simple and basic understanding of how LBP works. It's also a great place to start when it comes to learning how create mode works but in some ways it's even harder to create in LBP1 than LBP2. The second game is much more powerful but not too confusing if that makes sense.
I think LBP2 is more entertaining but it adds new features that might be a bit confusing. LBP3 is not really something I recommend to start with because it has a LOT of new stuff. I'm mostly talking about the create mode. As for story mode, LBP3 I've heard has been regarded as one of the easiest LBP games, so take that into consideration as well.