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.
-
CE and 4, without a doubt. But CE is the top dog here. ODST managed to [i]hugely[/i] improve Halo 3's rather awful sandbox by tweaking some of the weapons, note how all the titles I've mentioned so far as having a balanced sandbox have no dual-wielding (you'd almost think that there's a pattern there). The Spiker got a damage buff so it became a suitable alternative to the SMG if you ran out of ammo for it, same with the Mauler in comparison to the shotgun.