The claimants to the Parmesan throne Enrico and Giuseppe and their nephew Roberto II also never married.
Afaik, Enrico and Giuseppe were mentally handicapped.
What about Roberto II ?
After him, the line passed to the healthy sons of Duke Roberto I 2nd wedding to Infanta D.Maria Antónia of Bragança.
In Baden there was also a case of a mentally retarded prince Ludwig II.
The last two dukes of Brunswick-Luneburg also died unmarried Karl III and Wilhelm VIII.
the current Jacobite king of England, Scotland and Ireland Francis is a bachelor. He and his companion have never married. He'd also be King of Bavaria.
Olav V of Norway was not a bachelor but a widower when he succeeded so remained unwed during his entire reign.
Bulgaria's first sovereign Prince Alexander of Battenberg only married after his abdication so ruled as a bachelor.
Charles XII of Sweden never married so was a bachelor King.
The current King of Cambodia Norodom Sihamoni is a bachelor so he would be the most recent one.
Brother and successor to King Ludwig II, also unmarried.