Depends on the kind of file...

For white Text on empty black background I would use Photo-JPEG @100% (gives you more crisp borders than AIC)

If you need trasparent background you have to go Uncompressed with Alpha Channel...

Everything else would be Apple Intermediate Codec (best size / quality ratio in my eyes but as mentioned above not too good with hard contrasts like b/w text - though still better than MPG-2...)