I always find Zovirax clears them up the quickest (though I'm not sure if that's because I always feel them coming and treat the before the blister appears which definitely helps with the healing process).
Compeed isn't any where near as effective and I found they took a lot longer to clear up completely, but it did make them feel a lot more comfortable as they stop them forming a really hard scab which I find really sore, especially when they're right on the corner of your mouth and stop you from being able to open your mouth without them hurting).
So depends if you're more concerned with comfort or clearing them up quickly but with a nice crusty painful blister?!