My favorite is toddler, probably between 2-3.  My least favorite so far is 8-9.  They aren't babies, but aren't really "big" either. 

I wonder if this depends on a moms personality or if there are some "universal" favorite and non-favorite ages.

I think that the hardest stage is from 7 or 8 until 10 or so.  I love new baby and toddler, preschoolers and now teenagers, but it seems that the stage from 8-10 is tough.  They aren't as cute anymore and are still too childish to be peers.

We currently have children from 1 up to 13 (and one more due in July) so I get to see all the stages at one time.  I'm loving having a teenager!!!

I think my favorite so far is probably between 1 - 2. They're just learning to walk, talk and do all those fun things. They're still napping and are just so adorable. My least favorite is any age with tantrums and fits! haha :)
The only older kids I've had to deal with so far are my cousins and my step-daughter...so I haven't experienced the full blown tweens or teens yet.