Halo Reach has the best level design of the franchise, alongside ODST.
And by that I mean it has no flood levels.
I have to disagree that it's all about multiplayer. I can't fathom that multiplayer was given a lot of thought. It focuses on one gun. Imagine if Starcraft multiplayer only had one balanced unit. It'd be very competitive. But very repetitive. Weapon balance simply wasn't a consideration.