When you say you are an experienced knitter, are you looking for a complex pattern or are you just letting us know your skill level to keep in mind when recommending patterns?
If you're not looking for an overly complex pattern, THIS is one of my favorites when it comes to baby sweaters. It's a great basic pattern, so you can easily add your own touches (eg. different colored stripes, or perhaps a cable pattern down the front). The pattern is written for a newborn size, but easily increased ... I recently made one that was about a 6 month size by simply following the pattern as written and moving up to a size US 6 needle instead of the US 4 called for in the pattern.
Hope it helps! Good luck with your project, whatever you end up choosing!