whats best loadout on modern warfare.

Depends on your playstyle...

Recently I've been running the M13 with a .357 as a secondary, or using the 725 as a secondary with the Overkill perk. I use this class set as a pretty heavy attack based setup, and I would highly recommend it. If you haven't used the M13 yet definitely give it a try. To me, the M13 is just a better M4A1 with recoil control that is among the best of the assault rifles in the game.

For a support style setup instead, check out the M91 or PKM. Both are very effective at moderate to long range and have a high magazine capacity because they are LMG's. Seeing as though this is more of a support loadout, I typically use the JOKR or RPG as my secondary with this class.

Lastly, a marksman loadout can be useful on the more open maps in the game. I always have a sniping class in the game, and if you run a class with overkill you can have a marksman rifle AND a sniper rifle. This allows you to be effective at (almost) all ranges.