Dear Zoo by Rod Campbell. It's a classic for a reason. Let your child open the flap and say the name of the animal for them. If you are using baby signing then do the sign with them too. After a few readings leave a pause for your child to say the animal name themselves (but avoid doing a 'what's that?' quiz, a pause will do). For older babies or toddlers it is also a great opportunity to talk about describing words such as 'big' and 'tall'. Try doing an action as you say these words to support your baby's understanding of the meaning. If you don't already have this book you can buy it here
Eyes, Nose, Toes... Peekaboo! Use this book to support your child's understanding of body parts, Touch your child's ears, nose etc as you tell the story to give them extra opportunities to link the words with the meanings. You can buy Eyes, Nose, Toes Peekaboo here.
Baby Faces Peekaboo. Most lift the flap books focus on naming animals or objects. Peekaboo Faces stands out because it is a gentle introduction to feelings. Use the big mirror at the back for you and your baby to practise making faces in, and say the name of the emotion you are showing at the same time .You can buy Baby Faces Peekaboo here.
Fox's Socks: Part of the Tales from Acorn Wood series written by Julia Donaldson and beautifully illustrated by Alex Scheffler as always, this book is great for learning clothes vocabulary. After a few times reading this story see if your child can complete the sentence for you - the rhyme should help them. Try saying 'under the mat he finds his....' and see if they can say 'hat'. If you would like to buy it follow this link
So that's my top five, I'd love to hear whether you have found any lift the flap books particularly helpful for your baby; let me know in comments!