Interesting (in a good way) choices. Check out the Swede Pop group Amason's "California Dreamin" https://youtu.be/kYWXD4Ygyv8 It plumbs the darkness that the Mamas and Papas' version suggests.

Three covers that I think are as good as the originals, just different (more emphasis on the vocals): Robert Wyatt's version of Chic's "At Last I Am Free": Gene Clark's "If You Could Read My Mind"; The Unthanks' cover of King Crimson's "Starless."

Covers that I prefer to the originals:

*Rose Melberg's cover of the Beatles' "I Will" (difficult to find)

* anything by Hellsongs (since I'm not a big metal fan anyway, but even some metalheads like their lounge take)

* Billy May's Now Sounds version of The Toys' "Lovers Concerto," which is itself a sort of cover of Bach's (Petzold's) Minuet in G Major. Mays' orchestration is more Bach-like than Bach's. https://youtu.be/A4X4yEXtlkg

* from the above album, Mays' parody of "Strangers in the Night" If you don't like Sinatra's original (I loathe it), you should like this--stick with it at least until the Elmer Fudd imitation.