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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CE. Maybe the best weapon balance in a game ever. Each weapon had a role, but also disadvantages. 2's dual-wielding messed things up a lot, plus Brute Plasma Rifles. wat Even more redundant Brute weapons in 3. Reach had a lot of asymmetry in the UNSC/Covenant division, making some of Invasion unfair. 4 had some great stuff, but the Forerunner sandbox had a lot of missed potential.