Here's the boss zombie from the most recent session of my GURPS Zombies game. A combination of tight quarters and very poor rolls on a couple explosives meant this guy stayed up and terrorized them for a while. He relied on All-Out Attack (Double) to execute his Bellow Lungs and Mighty Roar in the same turn, partially for flavor and partially because it's a mean one-two punch. Unfortunately, the badass PCs and NPC ally were able to dodge the roar all but once. That was the only damage this guy landed, too, as he sent the NPC flying across the room to smash face-first into the far wall, breaking his nose.
I like where the zombie's resilience ended up. One PC uses an assault rifle (5d pi, RoF 15) while the other uses an axe and a hatchet (2d+7 and 2d+4 cut respectively), and they both wound up doing about the same amount of damage each turn after DR and Injury Tolerance. The poor NPC with his shotgun basically couldn't hurt this guy, though. He eventually switched to a pistol close-up and shaved a few HP off.
Humongous Enraged Infected (Boss)ST: 23 HP: 23 Speed: 5.00
DX: 10 Will: 8 Move: 7
IQ: 8 Per: 8
HT: 9 FP: 9 SM: +2
Dodge: 8 Parry: 8 (Unarmed) DR: 10
Grapple (10): Reach C. Often All-Out for +4 to hit!
Bellow Lungs (12): Range 10 yards (no range penalty). If hit, target takes (8d+2)x2 cr for knockback only.
Mighty Roar (12): Range 10 yards (no range penalty). If hit, target must make a HT roll (at +5 for Protected Hearing or +1 for Robust Hearing) or be stunned; he may roll HT each turn to recover.
Plague Bite (10): 4d-1 crushing + toxic effects as for infected. Reach C. Often All-Out for +4 to hit!
Traits: As infected (pp. 97-98 of GURPS Zombies), plus Berserk (Vicious) (9), Indomitable, Striking ST 15, and Unfazeable.
Mutations: Bellow Lungs, Mighty Roar, Redundant Organs, Thick Hide 3.
Skills: Innate Attack (Breath)-12.
Notes: Unliving (from Redundant Organs). Wound modifiers become Imp and pi++ x1, pi+ x1/2, pi x1/3, and pi- x1/5.