Well it usually depends on what map and gametype. I'll give you a basic layout for Team Slayer gamemode.

Team Slayer(Doesn't matter which one)
Primary- DMR for big Maps and mostly BR for CQB but you can use the Assault rifle, it's actually pretty good up close.
Secondary-Almost always Magnum I played with the Bolt Shot and the Plasma and they just dont stack up. If your playing Big Team Battle use the Plasma pistol, it'll help with taking out vehicles.
Grenades-Personaly I like the Sticky but it really doesn't matter. Damage wise, the Frag is the way to go. Plasma is good for Big Team Battle and the EMP/3rd(Can't remeber name)has the same effect on vehicles and people the plasma does (Can't remeber name).
Armor Abilities-It all depends on what map and gametype. I like Jetpack for Team Infinity and Health Regen for Regecide. Since you can sprint mobility isn't a problem if its a Big Team Map. Try using Hologram if your unsure, it's always helped me. The Autosentry is decent but not worth the SP at this point in the game. Promethean Vision is a pretty good ability if your a more conservative player. If you sit in the back of the map on Ragnarock or if its really intese CQB, it really helps.

Everything is up too you since I havent unlocked everything yet and the game's been out for a day. Goodluck.