CoC is a classic, deservedly so, and it still plays well for both one-shots and campaigns. Its flexibility for settings and eras is marvellous, and its Cthulhu-based resources are long, many, and mostly all useful.
I'm a sucker for Maque of the Red Death, despite the cheese in the official products, because I like the period, and I think it does Ravenloft better than Ravenloft does.