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.
I really like the current Ion right now, though that was only after the fight with Prime. I like the fact that he's such a powerful character, easily one of the strongest in the main DCverse but has such a massive flaw (the lead poisoning) and despite all his vast power (Ion entity) he still needs his ring.
In the same vein I also like Sentry, one of the strongest good guys in the 616verse, held back largely by his doubts and insecurities