its a hard question to answer without knowing your playstyles, but personally I use this on my MP5

stand off barrel-casket mags-stabilised stock

its hard not to get focussed on shot power, a stronger bullet kills more people right? but if you cant get your shots on target because the guns kicking like a flag in the wind then shot power is worthless, same with ammo loadout, if you die a lot then its fine, but if you tend to go for 10 kill rampages then more ammo = more kills, shot power becomes unimportant when your clip is empty

however my playstyle is long reach, large map shooting, but in a place such as sharqi (or other shorter range maps) with no shotgun to back you up then i can see a use for more of a CQ loadout of close in barrel+cqc ammo, youll die more so caskets not so good, and the ammo is more tuned mow them down assault rifle kind of power (those two on the default MP5 gives you 30 average per bullet up to 32m)

softpoint ammo is average all over - no big gains anywhere, giving up softpoint for cqc or high power ammo only loses 1 off the far end and adds more than 1 to your chosen end, its a tradeoff but one I like.

Ultimately theres no real best loadout, playstyle and type of map always is a factor, understanding those leads to better choices with attachments.

hth