Sloth
In terms of reliability, the sloth offers consistent predictability within certain parameters. One can rely upon a sloth to be slow. One can depend upon it to sleep extensively. One can trust that it will descend its tree approximately once per week for bathroom purposes. These are certainties in an uncertain world.
However, the sloth cannot be relied upon for punctuality, urgent assistance, or any task requiring completion within a human timeframe. As a postal service, it would be catastrophic. As a rescue animal, inadvisable. Its reliability is domain-specific at best.
Time
Time is the gold standard of reliability. It has never taken a day off, never called in sick, never slowed down because it 'just wasn't feeling it today.' Since the universe began, time has maintained its pace with mechanical precision that makes Swiss watchmakers weep with professional envy.
Every scientific measurement ultimately references time. Every contract depends upon it. The phrase 'as reliable as time' exists precisely because nothing else achieves this benchmark. Time may be cruel, indifferent, and impossible to negotiate with, but it is utterly, devastatingly reliable.
VERDICT
Time wins this category with the authority of 13.8 billion years of perfect attendance. The sloth's reliability, while charming in its specificity, cannot compete with a force that has never once failed to proceed forward at exactly the rate required.