I find that the best Weißbier doesn't come from Munich, but more so from the surrounding areas. Some of the best are:
Weihenstephaner (30 mins north, near the airport)
Andechs (a monastery to the south)
Hopf (small town to the south)
Ayinger (small town to the south)
I don't think you'll find these in the 5L kegs though. Best chance would be Franzeskaner.
For the 6 Munich breweries, they all focus on Helles(bright) Lager, and most of the breweries in Bavaria make either really good Lager or really good Weißbier, rarely both. Some only do one or the other.
I'd rate the Munich lagers:
Franzeskaner is made by Spaten brewery.
IMO Augustiner is the best by a mile. I think it's the most popular, and as far as I know they don't export it since they refuse to add anything to it for export. At home I usually drink the Helles Lager from Maxlrainer, which comes from a regional town next to a castle, and I live a little bit outside Munich now.