One man's opinion (obviously):

Bushmaster XM15-E2S - 5.56 mm is a ubiquitous round and likely to be available. 30 round magazines are good, and you can even go to 40 rounds for larger crowds of assholes.

UTAS XTR-12 12 Gauge shotgun - 10+1 capacity, and badass.

Ruger American 9mm - 9mm is the most common round, and 15+1 capacity is good in a closeup scenario.

Bonus: Smith & Wesson SW1911 in .45 ACP. Just because it's badass, and one sweet piece.