This depends entirely on the child. Personally, I have found the 18-month mark to be tough!
Age two is infamously named the “terrible twos” since tantrums and the need for control increase. I like this age, though, because his personality is coming out, my little one has good enough language to communicate, and every day is hilarious!
Many parents find that age three (being a ‘three-nager’) is even more challenging. It all depends on the child.
Some research suggests that parents struggle most with middle schoolers when puberty is in full swing (source).