The MP7 wasn't even the best gun in its own game. The ACR easily beats out the MP7 for competitive play.
The MP40 from World at War would probably be the best in the entire franchise, comparatively.
Other honorable mentions: UMP45 in MW2, FAMAS/M16 in MW2, FAMAS/AUG/AK74u in Black Ops
More honorable mentions: CoD4 M16 or MP5, PDW/Skorpion or Auto FAL in BO2.
I think the MP40 was probably the strongest. Then again, it might be because it was only significant compared to the other guns in the same game. Who knows how it would do against guns in other CoD games.
Xbox Live: Eighth Element || PSN: Echo_Resonance ~People will always nitpick where they have bias.~
I think the MK48 in this game is one of the best guns in the series. Low kick, large mag, and high, high damage make the thing the most deadly LMG I've ever had the pleasure of using in a CoD game. Seriously, if you want easy mode, slap Target Finder on this thing and watch in awe.