I echo the others, CVIII, hands-down! I'm pretty much as casual a gamer as can be, and DXC, even the alternate stage, didn't take that long to beat(or get used to). CVIII, to this day, is still one of the only CV games I've never actually beaten. Particularly the last couple or five stages nearing Dracula are, IMO, some of the most nail-biting stages I've ever played in the entire series.
Before I even got a chance to play the original RoB, I used to hear it was also a pretty tough game, but I played through that one in one sitting. Easy pie! DXC, the arranged version, took a little longer, but once you get used to some of the boss patterns, it's simple stuff. Of course, SotN was always simple.