I've always used the simple Jiffy peat-pucks, and have pretty much 100% success each time, give or take a stubborn ones every once in a while. Just remember to wring them out good right after they soak up water and expand, and it's also a good idea to take the netting off right from the start. They won't crumble as long as they're not allowed to dry out.

I've also heard lots of great testimonies for Rapid Rooters, but I've yet to give those a try.