My favorite accessory is really a combo...the rubber skin and the SEIDIO skinned holster. They both work well together. The skin protects my BB because I have dropped in on several occassions. The skinned holster (with magnet) is sleek and not a bulky holster with flap.
I just got my curve and i only bought the rubber slip on case thingy and i love it its black and i want to get a pink one soon, and then i hope to get the screen protectors soon
My solid aluminum case/cover with the belt swivel clip and the 4 GB SD Card as a runner up. The Motorola S8's are the the top of my lust, but I don't own them yet.
The Otter box case - I have dropped my Blackberry twice from over a meter and it's still fine but I don't want to chance it third time. The case is pretty indestructible.