In the Campaign I'll gladly use almost anything, because the weapons are well balanced. My favorite weapons tend to be the mid-long range weapons like the BR.
In Warzone, my favorite weapons are the AR and BR variants that have scopes and the Hard-light ammo. Especially the AR variant, because I can get that variant around the time my team takes our first base, and I get to keep it with me for the entire rest of the game. The BR variant I'll usually pick up later when I'm re-spawning anyways. They're both great for mid-long-range firefights, as well as for taking down vehicles.