Oops.. I lied. I voted "none".. but I realized that I got a 1 gig SD card free with my phone. So, I have a media card. And I just ordered a holster/belt clip.
I would have to say the case, although I was dissapointed that I had to go out and buy it instead of recieving one for free as I had for prevoiuos blackberrys that I purchased.
Both SEIDIO batteries are by far my favorite accessories! The 2400 mah one is a beast, but it lasts just about forever. I use the 1250 mah one the most, it's a hair larger then oem and came with it's own cover for both. Also having a good screen protector is a must if you don't like covers/cases.
I have a 2 gig MSD card, Sena leather case, Jawbone BT headset, and a desktop charger, as well as a couple car chargers. I would be hardpressed to pick just one...the Jawbone is awesome and I use it the most so I guess I would pick it as my #1...although the base charger is nice; I feel like I am putting my BB to bed at night, ha!
I like my OEM skin because it allows me to (gently) toss my BB into "the black hole" aka my purse, without getting worried about nasty scratches marring the oh-so-shiny chrome side rails of my 8830. I love my BSE screen protector for the same reasons...only this time protecting my screen from scratches.
And I'd be loss without my 2GB microSD card, which allows me to store almost 100 eBooks in my BB, with plenty of space to spare.
I have a 4 GB MicroSD card (filled with music), Plantronic 665 Bluetooth Headset, and a leather RIM case. I asked for the chrome trimmed charging cradle for Christmas though.