If you have the money, K&N FIPK. I got it for my 5.2L, and never looked back. But it depends. If you like deep water, get a snorkel. If you want the space for onboard air or a second battery, then go turbocity or ebay has a bunch of smaller stuff. The reason people get good performance has next to nothing to do with replacing the box filter. The stock tube that sits on top your throttlebody is whats restricting airflow.
If anyone claims to be better than the K&N FIPK, ask for a third party dyno test first =)