I barely use a quarter of the cast, but here's some automatic loses when I even try to fight them.
1. Brad Wong
2. Rig
3. Alpha-152
4. Sarah
5. Akira
6. Mila
Then again, I only base this off of my experience fighting them, and the ONLY experience I have is the AI. I really hope to play online a lot more once DOA5U hits now that I can finally tether my phone's Wi-Fi connection to my PS3.