My favorites are Ruby, Carmen, Wendy and Patty. My son really likes Peanut. I don't think you can go wrong with any of the Peppies though. They are all really cute.
I have a hard time liking most peppies because they're so over-the-top but Ruby and Tangy are great! I like Tangy's citrus thheme (esp the head leaf!) and facial features and how she doesn't look quite as girly as some other peppies. Ruby has a really cool space-themed house which is why I got her in the first place but she's turned out real nice in other ways too.
I'd recommend Carmen and Cheri because I've had them before and think they're great! I'd also recommend Epona and Piper I never had them but I like how they look.