Although I love CE's weapon sandbox and I consider it to be one of the best in the series, I'm actually gonna go with Reach on this one.
I just loved the Pro Pipe. And the Concussion Rifle. And the Needle Rifle. And the Focus Rifle.
I miss the Pro Pipe and Focus Rifle.
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I enjoy Halo 4's Sandbox simply because every weapon has a purpose. A role. Unlike Halo 2/3 where you have half-assed weapons that aren't even touch half the time. Plus, the inclusion of Promethean weaponry is awesome.