We love the Cachamate Peperina flavor. Right now, we just have plain hierba because they were out of the flavor, but don't knock the flavored 'til you try it!
I don't drink mate, but local connoisseurs swear by yerba Canarias. Found everywhere in Uruguay, and in a few BA supermarkets. I take about thirty half-kilo packages every time I return to BA - they make great gifts.
Canarias from Uruguay is the best. You have to get the 'mezcla Serena'. Everytime I go to Uruguay I make sure to pick up a few kilos. The best yerba I've have from Argentina is Playadito and the worst would be Rosamonte.