Well there are a lot of characters I like, for example, Chief, Arbiter, Sgt. Johnson, Jorge, Lasky (if he was more developed), Cortana, Guilty Spark, and Atriox.
However, recently, my most favorite character has actually been Exuberant Witness!
Its just something about her cheery and carefree yet 'can do attitude' that is hilarious and endearing. Making me wonder what thing she'll say next! She feels like a worthy 'replacement' for 343-Guilty-Spark (even though actually still alive) as he was fairly cheery yet serious in his own way as well.
Oh, also, you DON'T want to mess with her. With her tendency to cage anything fascinating and rage over others taking her things (looking at you Cortana) it adds a incredibly strong interior to the seemingly sweet exterior of the character.