Ian McKellen. I have an audiobook of him reading The Odyssey and even the endless descriptions of banquets are great.
Peter Lorre!
Ken Nordine
Juliet Stevenson
Jeremy Irons
Diana Rigg
John Gielgud
Albert “stop! that! train!” Finney
Alan Ladd. No really: everyone remembers him for wearing elevator shoes, but he had a great voice.
John Hurt
(not an actor, but my favorite Southerner) David Brinkley
(also not an actor, but if you ever heard him you never forgot his voice): Senator Everett Dirksen (R-IL)
Does Garrison Keillor count as an actor? He plays a lot of roles on APHC and has been in some movies.
My sister had a cockatiel that would dance and preen whenever GK’s voice was on the radio.
Speaking of APHC, I used to have *such *a voice-crush on Ivy Austin for her work on that show. A seriously lovely voice.
And a small correction: Jeffrey Tambor’s name is u-less.
Will Arnett. His regular speaking voice is perfect. He can also control it to do any type of voice in commercials. You hear him a lot more than you realize.
I had to look up the name, Iain Glen. He plays Ser Jorah on Game of Thrones.
Another for Stephen Fry. I love listening to the Hitchhiker’s Guide because of him. Another is Nigel Planer when he’s not being whiny. His audiobook reading is wonderful.
Sam Elliot. His voice makes me go weak at the knees.
Miranda Otto.
Dolly Parton - just love her speaking voice.
I hate to say it but Kevin Costner has a very honest sounding voice.
Unless age has ruined her voice, Jane Fonda. Her voice was so beautiful that I noticed it as a horny teenager watching “Barbarella.”
Yup.
Just remembered…
Julian Barratt
Simon Pegg
As a bonus, they sound very similar to each other at times (not their accents but their voices).
Nice call.
Sharon Stone was the narrator for the kids’ show Harold & the Purple Crayon and… well, it gets my Purple Crayon going.
Lindsay Wagner also has/had a soothing-yet-sexy voice.
Someone already mentioned Benedict Cumberbatch, because someone always mentions him when this comes up. 
I don’t think anyone’s mentioned Peter Davison. Which is a shame.
Not an actor, but Trey Anastasio (Phish) has an incredibly soothing and lovely speaking voice. I didn’t even notice it until one day, my mom, of all people, said, “I could listen to this man read washing machine assembly directions.”
He has a terrific smile as well.
Howard da Silva, character actor from the 60s.
I love Franka Potente’s voice (Run Lola Run).
I always thought John Vernon (Dean Wormer) had great timbre