It's a respectable entry in a storied franchise. Whether or not it's the best or the worst is too much work for me, IMO - it's in the discussion with the other great games in the series, and that should be enough.
If I had a criticism, it would be that the big team maps are too cramped. Ragnarok is great. Exile has so many one-shot kill weapons/vehicles that it gets pretty frustrating. The rest of the maps are pretty small.
Maybe I've just gotten too used to Battlefield 3's maps, but the Halo 4 maps all seem really small.