I think that is obvious

Caine has his morals(and barely qualifies as a villain), Oazi can be killed, Rick can be contained(for a bit), and Bill can be tricked and erased, but not permanently killed. My money's on Bill.

1. Bill Cipher

2. Rick/Ozai (depends if Rick gets prep)

3. Ozai/Rick

4. Caine (so far)

Okay, do I even need to say anything at this point?