Like Frank Sinatra & Elvis, The Beatles & The Monkees, and Chaplin & Keaton, I think everyone has a favorite when it comes to Bela Lugosi & Boris Karloff. You can like them both, but usually just one has a really special place in your heart. I love both of their movies to no end, but if the Boris Karloff DVD collection and the Bela Lugosi DVD collection were both dangling off the edge of a cliff, I know which one I would rescue. Sorry Bela*.
While I love almost all horror movies from the 1930's and 1940's (Val Lewton movies especially) Boris Karloff's performances always stand out as my favorites. A dedicated thespian, Karloff acted for nearly 20 years before being cast as the monster in Frankenstein in 1931. I think that his association with monster movies belittles what a great actor he was (not that great acting didn't exist in monster movies, but you know what I mean.) He brings a softness and depth to even the most ruthless, one dimensional roles. A mummy played by anyone else would be a body wrapped in linen, but Karloff makes it a character.
My favorite Karloff performance is in The Body Snatcher. Karloff plays John Gray, a grave robber who supplies stolen corpses to a slimy doctor, played by Henry Daniell. It's also my favorite scary movie, one that gives me chills (and often nightmares!) every time I watch it.
In real life, Boris Karloff was nothing like the villains and monsters he played onscreen. From what I've read, he seems to have been the sweetest man in Hollywood -- donating his time to cheer up disabled children every Christmas in a Baltimore hospital and recording numerous children's records over the years. Probably his most famous effort for children was his narration of The Grinch in 1966. In a way, The Grinch is almost like a symbol of Karloff, a combination of the man and the character. The monster with a heart of gold.
*Luckily Bela Lugosi & Boris Karloff did star in quite a few films together, so there would be some overlap in my Karloff DVD collection :)