I have heard of Cassius and Oliver, of course.
Personally, with Cassius especially, I love the intimacy he has across the fourth wall (even if he doesn't have the same with others characters). I feel like I've known him for a long while just from the intricate detail put into his posts. Everything about him has a reason, even down to why he wears sunglasses and how the strength of his accent changes with his moods. His backstory just adds to the intimacy; it gives a whole lot of sympathy to him without it being horribly cliched or 'been there, done that'. His history just seems to flesh him out more than anything else could and gives sense to why he is who he is. Plus, his art is fabulous.
Oliver I know less but I immediately fell in love with him. He's one of those scruffy-haired, supposedly naive adult-children that tugs on your heart strings. Yet, he's got so many layers that I'm not entirely sure I will ever find out his true motive until I'm meant to. He's an adorable evil mastermind and his 'pet dragon demons' just add to the mystification factor surrounding him. I mean, I think I voted him 'Favourite Male OC' in the last KoE choice awards after roleplaying with him in a grand total of two threads. He's genius. I need more Oliver.