There are different ideas of what's "best".. for some it's raw power, some are more interested in running quiet, maybe even fanless.. Some must stay with a single slot, others can spare the space for a double-wide card.. Many things you could use to help decide.
Ther's a guy in the Netherlands who seems to know a little bit about RadeonHD on OS4, and he was kind enough to put a webpage together to answer your question.
http://www.hdrlab.org.nz/projects/amiga-os-4-projects/radeonhd-driver/radeonhd-card-recommendations-for-amigaos-4-x/I see that page was last updated just a few weeks ago, so the information should be pretty fresh.
