Which Mission is the best for grinding the most XP?
I would say the first, but if you’re also wanting to master commendations, then I would suggest replaying each mission as often as you can.
Is he right? The first mission?
Every Spartan Ops Mission will give you around 3k of xp depending on your commendations.
The XP is based off the number of people on a mission, usually with four players I get around 5K a mission, after that its just deciding which type of enemies you prefer to fight and if you want vehicles or just infantry fighting.