The Otter Box Case
is about the best protection you can get. The tradeoff for this superb protection is the added bulk. It basically wraps your BB in a hard plastic shell.
My personal preference is for a naked BlackBerry in a Slip-In Sleeve
. No clip, great for slipping in a coat pocket or just for carrying in your hand. No top flap so it's easy to retrieve the BB quickly when needed.
If I'm carrying my BB around the house or using it at my desk, I like a simple silicone sleeve. They're available everywhere, including AT&T stores. Like This