The below is my opinion:
Best VERY CHEAP starting FIGHTING Zombie: Construction Worker, $250. Replace ~L9 when Corporals become available.
SERVING Zombies:
Free: Yoga Instructor: 4 speed, 5 power, tips 3
$100: Politician: speed 4, power 3, Tips 5
$200: Nerd: speed 7, power 2, tips 4
$750 Mathlete: Speed 9, Power 4, Tips 3 —-can be a bit of an all-around zombie, but expensive early on. Hits FAST if not hard, but low energy.
Replace servers with CIOs ~L10: $1500, Speed 8, Power 3, Tips 8.
Beyond those (corporals and CIOs), you're talking zombies that cost toxin.