Global Style are the best ones i've used. There is heaps of variety in the shapes and designs. They were good quality stickers as well. I think they may be a little more pricey them some others though.
The worst i've ever bought were from 'that's mine'. They were the biggest waste of money. Half of the stickers wouldn't ever tear off the backing sheet so they went to waste. Also the designs of the stickers are nothing to rave about.