Oh, you got me curious. I was told a few years ago by a TCM practictioner that in general the antioxident content starts with the lowest being black tea, then green, then red, and white has the highest. I dont know if the research has changed much since then. I did do a super quick pubmed search on rooibos. I came up with 3 pages of hits. The only one that really got my attention this time was about how the fermenting process will reduce the amount of antimutagenic and antioxidant power. Here's a copy of the abstract.

J Agric Food Chem. 2001 Jan;49(1):114-7. Links
Influence of processing stages on antimutagenic and antioxidant potentials of rooibos tea.Standley L, Winterton P, Marnewick JL, Gelderblom WC, Joubert E, Britz TJ.
Department of Food Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa.

The antimutagenic and antioxidant potentials of rooibos (Aspalathus linearis) tea samples, collected from each of its major processing stages, were evaluated according to the Salmonella typhimurium mutagenicity test and the hydrogen donating ability and superoxide anion radical scavenging assays, respectively. Ten random samples were collected before and after fermentation, as well as after sun-drying, sieving, and steam pasteurization. Results indicated that the fermented tea had a significantly (P < 0.05) lower antimutagenic and antioxidant potential than the unfermented tea. Of the different processing stages, the most significant reduction in the antimutagenic and antioxidant property of the tea was found during the "fermentation" step. Sun-drying, sieving, and steam pasteurization also reduced the antimutagenic potential of the tea, although not to the same extent as the first processing step. The hydrogen donating ability was significantly increased after steam pasteurization in comparison to those of fermented and sun-dried tea. Pasteurization did not affect superoxide anion radical scavenging in comparison to fermented tea. Differences seem to exist in the antimutagenicity and antioxidant potencies of the tea sampled at the various stages during processing. A possible role of tea polyphenols in the antimutagenic and antioxdant activities of the tea is suggested as processing caused a significant reduction in the total polyphenolic content.

PMID: 11170567 [PubMed - indexed for MEDLINE]