I know this is all a matter of opinion, but when I see people talking about Heineken, Corona, Guiness, and such as being their favorite beers it makes me wonder whether or not they have even tried any other kinds of Pilsners, Mexican lagers, or stouts.
To me, most of the beers you can find in an everyday grocery or convenience store tend to be on the mass marketed and overrated side.
I do know a few people who have tried lots of different beers and still call Guiness their favorite, but usually when someone says [insert ordinary beer here] is their favorite beer and I introduce them to something along the same lines that is WAY better they are blown away. Next time you order a Guiness check and see if they have a Sam Smith's Oatmeal Stout, or a Young's Double Chocolate Stout. You kight be pleasantly surprised....
Don't even get me started with Corona. I've tried all of the Mexican imports and purchased several exotic beers IN Mexico and I can tell you that Corona is absolutely the WORST beer brewed in that country. If you like that flavor do a Dos Equis lager or a Pacifico. Bohemia and Negra Modelo are tasty too.
Don't get me wrong....most stores sell Sam Adams, and some are beginning to carry some of the better imports and micro's, but as a general rule it's a good idea to check out a store that specializes in beer and ask questions. Most of the time the guys working there are beer geeks and can suggest multiple alternatives to the mass produced stuff that will rock your sox.