Normally Easyjet are pretty good, where are you thinking of flying from/to. If its LGW we dont see many cancellations. The worst problem with Easyjet is normally winter weather disruption.
Make sure you check in early, they are strict with latecomers and dont let them fly.
Air Southwest are a much smaller outfit with only a few flights a day from LGW and have a reasonable reputation, but with little back up.