Genetically engineered and modified plants are your best defense against zombies. Though difficult to obtain and slow to reload, the "corn cannon" is undoubtedly the most effective deterrent, with the "blue cabbage catapult" a distant second.
I seem to do my best against zombies while running with a "pack-o-punched" RPK, or HK21 with an upgraded ray gun as a backup. Made it to level 30 solo with that kit. Only thing that stopped me was running out of ammo.
Or Bill Brasky...