Ochakovo: The most widely sold kvass in Russia. Very strongly carbonated, but it is properly sour - they claim to have some kind of "double-fermentation technology" but I think it's a marketing stunt, since all kvass is technically the product of double fermentation: alcoholic and lactic acid fermentation. It's hard to tell the finer nuances of its taste because it's so fizzy, but as I said, it is sour, and the aftertaste is not cloying. It's mostly okay - I tend to think of it as a baseline for determining if a brand of kvass is shitty or not depending on whether I like it more or less.
Nikola: The most popular kvass abroad. Less sour, less fizzy. The taste itself is pretty much the same, but they add more sugar, so it acquires a sticky and unpleasant aftertaste. I don't like it much. I will drink it if no other opportunity presents itself, but eh.
Hlebnyi Krai ("Land of Bread"): This is where this shit starts getting interesting. It's malty. God, I have no words to describe how malty and bitter it is. Black as the horriblest fucking sin, too. Probably my favorite factory-made kvass, with the possible exception of...
Tsarskie Pripasy ("Tsar's Larder"): This appeared on the market a short while ago. It's terribly expensive for a kvass - twice as expensive as Ochakovo. However, it has a very interesting taste: it's both sweet and sour and tastes faintly of raisins, though I didn't find them on the ingredient list. This is strange, because historically, bread kvass was often brewed with raisins. It is also unfiltered, so it's smoother to drink, and it's more mildly carbonated than most factory kvasses. I dunno if that excuses its price, however. They also make a black-currant kvass, but that's another story entirely.
Buket Chuvashii ("Bouquet of Chuvashia): I only tried it once and haven't been able to find it since. This is proper, honest, God-fearing kvass. Very distinctive rye taste, great smell, and none of that pansy "let's make it so fizzy you don't notice it's shit" tricksy business. Comrade P. got it somewhere, gotta ask him where.
Russki Dar ("Russian Gift"): Very generic. The taste is rather bland: it's not bad, there's just not much to say about it. It's like a music performance that manages to avoid making any off-key notes, but is still forgettable as hell. Cool bottle design, though, gotta give'em props for that.
Russki Kvas ("Russian Kvass"): Even more generic. It's like it had a competition with the above entry about who gets to be more unremarkable. Even the design of the bottle is similar.
Krushka i Bochka ("Mug and Barrel"): Bready, but also overly fizzy. Smells pretty good, though. I hear it's exported abroad to the UK and wherever. Dunno about that.
Street vendor kvass: Sold from yellow metal cisterns on wheels. Usually resembles Ochakovo, but breadier and less fizzy. Very traditional, has been sold since God-knows-when in the USSR.