Ditto Frog and Toad, Magic School bus, Paddington, Little Bear; Beatrix Potter

Madeline - was a big favorite of my son at 2.5 years old.

Curious George

Mr Men/Little Miss series

Henry and Mudge series

Fly Guy series, a bit aimed at reluctant readers and boys though, but they are funny and captivating.

Anything by Richard Scarry

Barbar

Anything by Maurice Sendak or Tomie DePaola