What is the best weapon combination in Modern Warfare 2?
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There really is no 'best' weapon combination that works for all players. Each player tends to use guns very differently based on their reaction time, ability to handle recoil, accuracy, whether they're color blind, their connection to host, if they're host or not, the amount of blood in their caffeine stream, etc. You will find that different guns tend to be better for you at different times of the day.
That being said, there are guns that are considered 'Top Tiered' and recognized as being some of the best guns in the game (dare I say the 'overpowered' word?). Here is a list of guns that will get you yelled at for using (so they must be good right?):
Nerd Rage Inducing Winners
This is in no way a comprehensive list, but feel free to add a grenade launcher or commando on any of these to make them even more powerful
ACR (with a red dot or holographic)
Scar (Silencer, red dot, holographic, or FMJ)
UMP (Silencer, red dot, or holographic)
RPD (Grip or Grip + FMJ: This one won't draw the 'overpowered' comments as much but does as much damage as a scar and has 100 bullets... The trick is being aware of its slow pull up time and compensating for that or using slight of hand pro)
AA-12 + Grip
Spaz + Grip
G18 + Akimbo
Ranger + Akimbo
M93 Rafica + Akimbo or red dot/holo
These are considered the most overpowered guns in the game. Your original question was a little vague but if you're looking for a load-out as well let me know (perks equipment etc).
Hope this helps and good luck with the wave of nerd rage you will experience shortly.
This is an opinion, but the UMP with a red dot, lightweight pro and slight of hand is amazing.
Depending on what mode and skill level, the answer varies.
UMP + Silencer, M9+silencer, claymore, flash, scavenger, cold blooded and ninja for hardcore
ACR/AK + FMJ, AA + Grip, C4/Claymore, Stun, Scavenger, Stopping Power, Steady Aim/Ninja for normal.
Those are my preferences