I get asked this question a lot, and I've defended quite a few cases where the only evidence against my client is testimony from someone else. The answer is yes, for most crimes you can be convicted solely on witness testimony.
But don't lose hope! There are many ways to defend a case where the only evidence is witness testimony. At the core of this defense is a thorough investigation of the witness's story and a ruthless cross-examination focusing on inconsistencies, impossibilities, and biases.
The case is a "he said, she said" situation, and it's important to make the jury believe the witness is lying.
If you're facing a case with only witness testimony and no physical evidence, call me at 404-909-7795. I'd be happy to discuss your defense options.