The cards that seem to most consistently perform well as boot drives are Sandisk class 2 and 4, 8GB and 16GB sizes. If you're able to benchmark the cards you have, the telling statistic is 4kb random writes. Most cards have 4kb random writes 10x or 100x lower than the above mentioned Sandisk cards.
Class itself is irrelevant to using a card as an Android boot drive. Even some Sandisks in the above class and size don't bench well for 4kb random writes, but those cards have been overall most reliable.
Source:
http://forum.xda-developers.com/show....php?t=1005633