I will always love playing as Arbiter, but the Chief missions are by far the more enjoyable ones(IMO). We all (usually) love the Chief, and his missions, (especially High Charity... man what an awesome mission) were some memorable moments.
I wouldn't say Arbiter's missions were unappealing, they just weren't as exciting as Chiefs, at least up until the last two missions. However, his plight throughout the game, seeing his brothers struck down and his world crumbling beneath his feet, was epic to watch. Arbiter actually was probably one of the best surprises we ever got playing a video game, seeing that other side, understanding their world compared to ours. Kind of makes me torn between this.
I'll choose the Chief missions for the fun, but Arbiter any day just because I love him