Lee Ingleby makes the series for me.
Disagree with the character's stance on many if not most things, but I love the way Lee Ingleby plays Bacchus. Entertaining, rounded, refreshing and strangely likeable (or at least understandable).
The inadequate, conflicted, tory voting, book-cover judging, adolescent, petulant version of the things that made Gene Hunt such a great character in Life On Mars. Almost without fail he makes every potentially dull interaction or set-up into something more entertaining, nuanced, fraught and multi-layered.
Along with the brilliant 60s references, stories, look, sets and sensibilities, it's the thing that keeps me most interested about every new series announcement. In the same way David Suchet, John Thaw and Jeremy Brett did for Poirot, Morse and Holmes. You love the characters and trust them to bring entertainment and a rich layering to almost every story, no matter what the direction.