I like homemade the best because I can get more gooey and less cake. But if I need to buy it, I'll go with http://www.mcarthurs.com/ because they use a square pan and the edges get perfectly chewy. Don't bother with a grocery store version of the cake. Just grab some gooey butter cookies at a grocery store if that's the only option you have.
I would agree with Irstl. McArthurs is my favorite bakery but I prefer it homemade. Some while ago, the Post-Dispatch published a restaurant recipe request for gooey butter cake. It was a more upscale restaurant (? 111 Mississippi) but when asked for their recipe for GBC, they gave the traditional cake mix, cream cheese, butter version. Don't know if that's what they actually serve (sometimes restaurants don't actually give their "real" recipes in that column) but I thought it interesting.