it depends on what you are going to use it for and if it will be your only compressor or not.

RNC generally gets good reviews all around.

Focusrite seems to be one of those love it or hate it brands - i've read good things about the Penta for vocals.

i would tend to avoid lower priced tube compressors. the warmth and fatness they advertise is often really just noise and distortion. but higher end tube and optical compressors really do sound nice and warm.