Pigeon
The pigeon's reliability record spans an astonishing five millennia of documented service. Ancient Mesopotamians employed them for military communications around 3000 BCE. The Greeks announced Olympic victors via pigeon. The Rothschild banking dynasty built a fortune partly upon pigeon-delivered financial intelligence. During the Siege of Paris in 1870, some 150,000 official messages reached the city by pigeon post.
In both World Wars, carrier pigeons demonstrated reliability that bordered on the miraculous. Cher Ami, a hen pigeon in American service, delivered a message that saved 194 lives whilst having been shot through the breast, blinded in one eye, and left with a leg hanging by a tendon. She completed her mission. The pigeon does not call in sick. The pigeon does not suffer existential crises about whether its abilities make it too powerful.
Superman
Superman's reliability presents what actuaries would term a mixed risk profile. When functioning optimally, he can move at speeds exceeding light, lift objects of virtually unlimited mass, and survive conditions ranging from deep space vacuum to the Earth's molten core. The theoretical upside is essentially infinite.
However, his operational parameters include numerous vulnerabilities. Kryptonite of various colours can weaken, corrupt, or kill him. Magic bypasses his defences entirely. Red sun radiation strips his powers completely. Mind control affects him with embarrassing regularity. Most critically, he has died on multiple occasions—a reliability metric that would concern any service provider. His returns from death, whilst welcome, do not inspire confidence in consistent availability.
VERDICT
The pigeon's 5,000 years of uninterrupted service against Superman's multiple canonical deaths and persistent vulnerability to glowing rocks establishes clear superiority. Reliability victory: Pigeon.