The Smart Bottom's 3.1 is my favorite diaper for my 16 month old right now! Not only is it organic and all natural fibers (no microfiber!), it is really trim too! It takes a few washes to fully prep it but I've only washed mine 2-3 times and started using them. Just know if you do this you'll have to change it more often until the absorbancy is at it's fullest with a total of 10-12 washes. Once it hits it's full absorbancy though... It's SUPER absorbent!
One size (8-35lbs) with rise snaps and a 10 layer 100% organic cotton snake style soaker (can be folded where you need the absorbancy the most).
I have not tried it on my 2.5 month old yet. But plan to do so soon!