Lets just get some basic facts straight: Most guns have the same damage as other guns in their class. See this site here: http://symthic.com/
So according to the site symthic, the G3A3 is the only assault rifle that asserts a significantly higher damage. it is 34 vs 25 at close range. All other rifles like the AEK, M16, M416, and KH2000, F2000 do the same damage.
Next spec to look at is DPM (damage per minute) or fire rate. This is calculated RPM (rounds per minute) * DMG (damge per round) = DPM. So the higher the RPM the faster you can deliver damage to the target.
Just looking at the highest fire rates you get the AN-94, F2000, FAMAS, and AEK-971 (Fire rates above 850)
Now, in my opinion the reason why these 4 guns are considered the "best" are because they have the potential
to deliver the most damage in the smallest amount of time. If BF3 were to include AI players, that analyzed the weapons for the best possible 4 choices, they would also choose these weapons. People will argue that the M16/M41 are better weapons, and this is because of their style of play. But based on numbers and logic, the four noted above will succeed, with the addition of the G3A3 for damage purposes (slow RPM though).
Next is where you factor in the play style to choose between the selected few. We look at recoil (vertical and horizontal) and spread of the weapon's fire. If you are a long range shooter, you want a small spread and a close range will use a wider spread. The recoil's are all different, so find one you can control or prefer to use. The differences may be minor as one pulls slightly to the left more than it does the right. Or more up instead of left. This is what the attachments are used to correct. If you notice the FAMAS has high recoil, so one would attach the fore grip to limit said recoil, and would not equip the heavy barrel as it just adds to the vert. recoil.
Again, players could argue different here as their play style may be extreme long range- with the use of a bipod. Then they want the heavy barrel for the increased distance, as the bipod takes care of the recoil.
Of course if you are a stealth player and like the Suppressor, you will be lowering the speed of the bullets causing them to do slightly less damage and have a shorter drop distance. I would not recommend the G3A3 with a suppressor, and just choose a Flash suppressor. They remove your gun flash, but not the sound. They do not effect bullet speed.
I am not saying the FAMAS or G3A3 are winners, as IMO the AUG-A3: RDS, Foregrip, FlashSup/HeavyBarrel fits my preferences, and seems to do quite well in all around game play/modes.
See a guide here about more in depth spec usage: http://forum.symthic.com/battlefield-3-technical-discussion/3840-foregrip-and-heavy-barrel-best-friends-forever/
I am saying you should use this method of thinking for choosing which guns may fit your game play, and then do field tests to decide between your choices.
And remember, everyone's opinion is their own, and you should not be interested in to conforming your own around theirs.