The Ethiopian is definitely one of the most popular but most don't consider it a "strong" coffee. It's a lighter roast but has a very complex, layered flavor profile.
The Sumatra is the darkest roast of the current offerings and has a bold, earthy flavor. This one is popular with people who are used to the coffees from the big chain shops. It's not roasted as dark as those though, since I feel they burn the best stuff out of the coffee.
If you're new to single origin coffees, I'd recommend that you start with the Guatemala Huehuetenango. It's a medium roasted coffee with a flavor profile that is more recognizable to many folks who have grown up on Central/South American coffees but I think you'll find it to be richer and much more flavorful.