The beer scene in Sofia is quite limited, though has improved dramatically in the past couple years (from a very low base).
Kanaal is a probably the best beer bar in town, although the newer Vitamin B is also solid. Both places should have some beer from the new wave of Bulgarian craft brewers, as well as other interesting Eastern European breweries (and interesting craft beer in general).
That said, based on my most recent experience the quality level of the Bulgarian Brewers is still fairly average. So, I’m not sure there are any I would recommend above the rest. But there could be new brewers that have started up or others could have improved - hopefully you can find a bartender to provide some input.
There are very few breweries in Bulgaria, it is worth noting: http://www.ratebeer.com/breweries/bulgaria/0/34/
The bottle shops in Sofia are decent - E.g. the highly rated ones in RB Places - and you should be able to find a wide range of Bulgarian beer in them. By wide, I mean around a dozen or so! Unless, of course, things have changed in the months since my last visit. These bottle shops will also have good beers from Belgium, the US, UK and Europe in general.