I have found at least two classes that I have found very useful in free for all.
I usually go 20+ kills and less than 10 deaths.
Primary: ump45(silenced)
Secondary: spas-12(fore grip)
Equipment: Semtex
Special grenade: flash bang
And since I run and gun I use
Perk 1: marathon
Perk 2: lightweight
Perk 3: commando
Pro on all of them is helpful
Then for a strong yet undetectable class I use...
Primary: fal(silencer and holographic sight)
Secondary: spas-12(fore grip and holographic sight)
Equipment: Semtex
Special grenade: flash bang
Perk 1: bling pro
Perk 2: coldblooded
Perk 3: ninja