Whats the best possible way?
I assume you mean level up your SR in which case playing the warzone modes with boosts earns XP the fastest.
Any Warzone playlist, BTB, and I’m not sure if this is still a thing, but I remember rewards being increased for Infection a few months back, but that might have just been to make it a featured playlist for a week or two.
If you’re like me and don’t really enjoy the frustration that Warzone brings, the featured playlists offer XP boosts when you play them.
Swat, putting xp boosts on as well as warzone.
alot of great tips thanks 
Boosts. Lots of boosts.
The fastest way to increase your SR is to just sit there and play heroic warzone firefight putting xp boost of any kind on every single game. You’ll get bored as hell, but it is the fastest way.
It used to be 12 man warzone quick cores in under 3 minutes. Pretty much all of the first 10 or 15 guys that hit SR152 (max rank) used this method to get there. A really good team could turn out 10 games of Warzone an hour with a boost on every game and grind XP really fast.
Of course now with the 6 man limit you’re forced to have 6 players on your team that don’t have a clue what is going on dragging the game out for 25 minutes no matter how good you are. Like I said, heroic warzone firefight. By far the fastest way to rank up.
Mythic WZFF on the weekend with xp boost’s! Win or Loss its decent
Play with a person or two with low MMR (to fudge the matching system) and run TRAIN! 
lol
Grind Warzone with friends.
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> Whats the best possible way?
Playing Mythic Warzone will get you a lot of xp, even if you don’t win. It also gives more xp than Swat and other game types.