The book I read right now for no other reason than to see the character development is probably Nova. Of all time? That's hard. For me it's been Batman, Tony Stark, Silver Surfer, Peter Parker, Ghost Rider but they've all had plenty of boring periods too. Rorschach was a high point, but short-lived. I'd probably say Morpheus (Sandman) for sheer consistency of being so interesting he's a must-read for his entire run.