Gosh, all of them for different reasons.
The first 6m, I found to be a gentle transition to Parenthood with lot of cuddles and I had more time to do stuff and go and do things that I'd wanted to do for years but was too busy. So much time going on holidays, weekends seeing friends, parties.
6-18m, it got much harder work (and slept worse than a newborn), but they started becoming so much more rewarding, and it was lovely to see them learning so much and getting mobility. They were just so cute.
18m-3: really hard work (still didn't sleep), but I loved/line seeing their in vocabulary come on, and the start of being able to have conversations. There's really none of the lazy lunches with friends of early parenthood any more and it s hard to even have a conversation with friends a lot of the time. But they are starting to turn into fun people.
3-4 (as far as I've got). No more naps means there is no let up, and my one never stops talking, though thankfully finally sleeping better. They are genuinely funny, and can be so loving and kind, and that's lovely to see. They can be genuinely great company, but they can also be deeply annoying. My one doesn't like it when other people are talking - the attention has to be on her 🙄
I guess for quality of life, I'd say I loved the first couple of months best. For spending time with them, I'd say around 2 was my favourite.