Try Kage Baker's novels of the Company (no, I don't get a commission; maybe I should look into that
In the Garden of Iden
Mendoza in Hollywood (published as At the Edge of the West in the UK)
The Graveyard Game
The Life of the World to Come
The Children of the Company
There is also an associated book of short stories: The Company Dossiers: Black Projects, White Knights.
There is time travel here, and cyborgs, and romance, and comedy, and some intereting social commentary. I don't know about "best series", but it is my favorite series.