I'm afraid there is no "right" answer. It depends on what you want to use it for.
The one thing I have seen, after testing over a dozen cards now, is the difference in random Write speed. Most cards are slow at this, but some (an example being a Sandisk Ultra class 10) is 3,000 times faster than most others. This same card however, is slow (comparatively) at reading contiguous data (just above 10mbs). A much cheaper class 4 Kingston can read just as fast (almost), and write data at half the speed.. but for random writes, it's thousands of times slower.. but it's much cheaper.
In day-to-day operation, if you're not a developer (lots of random reads and writes) and a mostly a consumer, any branded class 4 card will be fine. If you want to record HD tv in XBMC, choose one with a fast contiguous write speed (Transcend as mentioned before is a good one for that).
If you have a specific need, do more research. Other than that, buy a branded not-from-ebay class 4, or for large storage needs, a class 10.
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