Well, my oldest is just 10.5, but I enjoy them all more the older they get. Well, one is my least favorite year, so I like them better before and after that. I mean, it's one of the cutest ages, but it's so physically demanding and draining. They're mobile enough and dumb enough to be such a danger to themselves at age one. I do not do well with lack of sleep, and I don't sleep for the first couple of years. Not that I want my kids to grow up too fast, and I definitely savor the baby cuddles, but I'll be so glad to not have a baby/toddler.
Anyway, I didn't vote, because for me so far, each time, the hardest has been up to age two.