If you want to rank up quickly, then it's highly recommended that you play Chaos Moshpit and chose NUK3TOWN.
I learnt this from Natsu, he was correct about the mass XP that you can earn..
Domination is a great way to earn XP.
If you capture A/C and then go for B.. You can earn mass XP just for defending the flags and throwing nades across the map.
Shoutout to Natsu for telling me about this!
I went up 3 ranks in 2 games