Thanks, Marley23, for making me feel positively ancient. 
I dunno who he is, but keep him far away from this generation’s Jonathan Brewster!
Nothing personal, Gramps (:D), but I was curious and trying to set some parameters. If Christopher Lee is in the same generation as Gary Oldman (35 years younger than Lee), than he’s probably in the same generation as Karloff (35 years older than Lee). If we want to toss that aside and give it to Christopher Lee, I won’t complain. I could listen to that man read the phone book.
John Malkovich
Wow, some great choices! I definitely would add Gary Oldman and Alan Rickman and John Malkovich to the list of contenders. Ron Perlman is potentially up there, I just haven’t seen enough of his movies to tell if I personally would include him.
I’m tempted to disallow Max von Sydow (definitely a great actor) for the same reasons that Marley23 (correctly, IMO) disallowed Christopher Lee. This of course brings up a sticky question; what, exactly, is the cutoff point for an actor of “this generation?” I said initially that Anthony Hopkins was one of my primary candidates, and he’s not that much younger than von Sydow. Christopher Walken started acting in the 50s, although he really didn’t start coming into his own until the 60s. I’m tempted to say that, with reasonable exceptions, we should say that the actor/actress had to start his/her film acting career in the 60s. What exceptions? Well, someone who technically began acting in the 50s but has been very active during the 70s, 80s and beyond would get an exception from me. I’d still probably disallow Lee because he really first hit it big in the 40s and 50s. That’s just a little too old to be on this list IMO.
I don’t know; it’s difficult to come up with a set of rules that I’m satisfied with here; acting careers span so many decades sometimes that someone like Lee or Sydow can have several phases of his career. Any suggestions for guidelines are welcome.
I hadn’t even thought of the female Karloff; of course, that would have to be more like “this generation’s Theda Bara” or something like that.
Hopkins was the first name to come to mind. He’s the best actor who comes closest to also being a “horror-genre” actor, although Oldman and Walken
do come close. Nicholson and DeNiro have taken pretty decent stabs also.
If Jeremy Irons had wanted to go that route, he’d have been excellent- hell,
he looks much like Karloff!
Aside from Hannibal Lecter, is he really a horror genre actor? For having played such monstrous characters as C. S. Lewis, John Quincy Adams, Richard Nixon, and Zorro? (Okay, maybe I’ll give you Nixon.)
Yeah, I was using the term “Horror” too loosely, BUT he does have a significant
body of Dark work.
Besides the Lecter Trilogy, Hopkins films that are at least “dark & weird”, if not
all-out “horror”-
MAGIC
AUDREY ROSE
BRAM STOKER’S DRACULA
FREEJACK
THE ELEPHANT MAN
HEARTS IN ATLANTIS
BEOWULF (upcoming)
TITUS (ya wanna tell me that Shakespeare’s Titus Andronicus ISN’T Horror? )
THE DEVIL AND DANIEL WEBSTER (yeah, I admit that’s reaching!)
THE BUNKER (Hitler!- and a damn fine scary one at that.)
THE HUNCHBACK OF NOTRE DAME
Donald Pleasance
But he was so sweet as Jamie in Truly Madly Deeply. That overshadows everything for me.
Julian Sands has increasingly become a weird creepyman in recent years.
Karloff? Karloff did not deserve to smell my shit! That limey cocksucker can rot in Hell for all I care! How dare you bring up Karloff? You think it takes talent to do Frankenstein? It’s all makeup and grunting!
Sounds like Bela needs his “medicine.”
I came in to mention him as well. After One Flew Over the Cuckoo’s Nest, he settled into horror and villain roles.
He’s a great actor. His Grima Wormtounge was amazing.
Not “this generation’s” as he is the late Donald Pleasance.
PULL THE STRINGS!
I don’t fully grasp the OP’s premise, as I haven’t really seen any of Boris Karloff’s work, but I’ll try.
I’d say it’s a toss-up between Sam Neill and Johnny Depp, although I’ll also at least mention Crispin Glover.
Sam Neill
The Omen (all of them)
Event Horizon
In the Mouth of Madness
Jurassic Park (?)
Snow White: A Tale of Terror (never saw it)
Dead Calm
Johnny Depp
Secret Window
From Hell
Sleepy Hollow
Ninth Gate
Astronaut’s Wife
Nightmare on Elm Street 1 & 6
Both have extensive horror / thriller experience, and both are solid actors.
Why just one?
Each & every Goth club in American has its local champ.
If they weren’t so sullen, I’d say organise a competitive league.