I second the sandisk brand. They not only make the majority of cards, but they design the standards for them that the others follow.
Cards can vary though, even within the same brand. That's why Sandisk, for example, sell about 20 odd different class 6 micro SD cards for a given capacity.
The class just determines the minimum write speed in MB/s. So a class 6 must be capable of saving data to it at a minimum rate of 6 MB per second. However, you tend to read from SD cards much more often than writing to them, and it's this speed that can vary massively. I have two Sandisk class 6s here for my HD camcorder. One copies data off it at 120 MB/s (which, by the way, is the most bandwidth you'll get out of nearly any USB2 chipset). Whereas another one copies over at about 30 MB/s.