The most common beers in Barcelona are Estrella, followed by San Miguel with Mahou a distant third, and while all are good and refreshing on a hot summer's day, they tend to be a bit gassy for me, and after three or four, I generally feel bloated.
The good news is that while Catalunya is famous for its Cava, it has also started producing some fine local beers over the last few years, providing a tastier alternative to the big two, and these are some of my favorites.
The most common of these cervezas is Moritz. Found in many bars, it offers a distinct taste that, personally, reminds me a bit of gin, making it my least favorite. Still, it's popularity cannot be disputed based on the number of people who have told me: it's the perfect beer for a hot, sunny day.
Glops was the first local brew I tried, and instantly fell in love with its malty, smokey, rich taste. Made in Poble Sec and launched in 2005, it's available in eight different flavors from a refreshing, easy to drink lager to a more substantial stout, with my favorite being the fumada. Not a common beer by any stretch of the imagination, it's becoming more available every year, so if you see it in a bar, definitely ask for one.
Rosita. Brewed just south of Barcelona in Tarragona, it comes in a lager, ale and stout. Rich and flavorful, serving it requires a soft shaking to mix in the natural sediments that fall to the bottom and it's to be sipped, not gulped. A recent entry to the Barcelona market, I've only found it at one of my locals, but given that it sells out quickly, I imagine it's only a question of time before it becomes a staple at some of the more popular drinking spots.
Finally, to try any of these, head down to La Cerveteca near the main post office off Laietana. A great place with a wide selection of beers from Europe, the States, Australia, and other countries, it's the beer-lovers dream.