Your question requires more specifics. Voltage source or current source? V/Hz or vector? Application? Environment? Etc.
"Reliability" from an engineering perspective is a function of the reliability of each component multiplied by the number of components, so the simplest answer is the "simplest VFD
with the highest reliability components." But each application has its own peculiarities, and what might work acceptably well on, say, a building ventilation blower would not survive long operating a material conveyor.