I think the Halo 4 Sandbox is the most balanced ever.
But clearly that doesn't mean its the best.
The only way to balance Halo is to do exactly what they did, come up with 3 tiers of weapons for all 3 factions, skin them differently, and change the particle effects.
I am really hoping for a big time overhaul for Covenant and Promethean weapons going forward, due to 13 years of existence* the Human base weapons will never change, the only way to get variety is add new UNSC power weapons, or revamp the enemies weapons.
* Vehicles suffer from this also, I dont know what they are going to do to make vehicle combat fun and unique again, but it needs to happen.
I know "Halo" to alot of people here is 4v4 Slayer on Symmetrical maps, and dont get me wrong, I love that.
But where Halo has a chance to provide something different from the competition to me is Objective gameplay, with team mates, in vehicles, playing cooperatively.
I know Battlefield has vehicles, but Halo has a special way of making the vehicles accessible, they need to really nail that niche next time around