Rules Cyclopedia all the way.
Dark Dungeons is a cleaner presentation, but has a completely new, different, unnecessary, and challenging-to-convert combat system.
With the RC, you have tons and tons of published material (through OD&D, BECMI, AD&D 1 and 2, 3rd party stuff, etc) that is very easily compatible. Dark Dungeons adds a weird middle step of conversion to everything.
The RC also has guidelines for Immortal stuff, it just doesn't go all the way. In my own use that hasn't been an issue because it's never been even remotely relevant, but obviously campaigns differ.
Disclosure: I use the RC in my regular weekly game and have been for some time now (multiple years). Flip the math if you want (I did, and added 5e's dis/advantage mechanic to boot), but the book really has everything you need.