(This could be considered a guide, so move the thread there if you feel it's necessary, I decided to post it here however because the guide section seems dead).
Hey guys, i've got quite a bit of experience with MW3 so I decided to share what classes I feel work best for me.
I've named the classes appropriately to what situation they work the best in. I'm going to assume you have most of the perks pro (Scavenger Pro and Hardline Pro are vital for these classes). Hopefully you can make use of them as well as I have.
"M.O.A.B" Class Setup
Primary: ACR 6.8, Two Attachments Proficiency, Suppressor, Extended Mags
Secondary: FMG9 Akimbo
Lethal: Frag
Tactical: Portable Radar
Perk 1: Scavenger Pro
Perk 2: Hardline Pro
Perk 3: Stalker Pro
KillStreak Setup: Specialist - Sleight of Hand, Quickdraw, Assassin
This class is probably my favourite. The ACR has next to no recoil and so it is very effective in almost all scenarios. FMG9 Akimbo is a good gun to panic swap to when you're in danger, as it is easily the best close range weapon there is. Portable radar can help show you when someone is sneaking up behind you and has saved my life a few off a MOAB countless times. Frag is more of a personal choice, the Lethal is not too important on most classes as they do not have too much of an effect on the gameplay in re-spawn game types. Scavenger Pro is vital for those long streaks, especially those where you are going for a MOAB; it means you can play defensively without having to run out of ammo. Hardline Pro gets your specialist bonus faster, and more importantly, those assists all stack up and get your MOAB a few kills faster than normal. Stalker Pro eliminates the need for Steady Aim/Quick Draw as you can just ADS around corners constantly. The specialist setup is designed to help people who are close to the enemy spawn, as they will need perks that speed up their actions when they try to win gunfights against multiple people.
Long Range Class Setup
Primary: MK14, Two Attachments, Rapid Fire, Red Dot OR Type 95 with the same.
Secondary: MP9 Silenced
Lethal: Frag
Tactical: Portable Radar/Tactical Insertion
Perk 1: Scavenger Pro
Perk 2: Hardline Pro
Perk 3: Marksman Pro
KillStreak Setup: Specialist - Stalker, Assassin, Sleight of Hand OR Assault - Predator, Reaper, AC-130
THe MK14 and Type 95 are the two best long-range assault rifles in the game, due to their accuracy, by far. Red dot on these weapons makes it easier to see your enemies - no annoying iron sights - and Rapid Fire speeds up the gap in between each shot, as they are semi-auto/3 round burst. Kick may be desired on the MK14 if you find the recoil too much. The MP9 also has the best range and lowest recoil of all Machine Pistols so it goes with the class naturally. Now, Long Range classes tend to be very camp-intensive classes - which is why this includes a Frag, the longest range Lethal (other than Throwing Knife, but that is difficult to use). Portable Radar can help if anyone gets too close, but the tactical insertion makes it useful to stay in your area even if you die. Scavenger Pro is vital for a class that is likely to be camped with as you will not have to run around for ammo, and Hardline simply gets the killstreaks much more easily for you. Marksman is vital for long range classes - the enemies can be seen MUCH more easily, though assassin pro will counter this. The non-passive Assault killstreaks are useful as you will not be doing much moving around anyway, so things such as the AH-6 will not be useful.
Medium Range "M.O.A.B" Class Setup
Primary: MP7, Two Attachments, Suppressor, Extended Mags
Secondary: MP9 Silenced
Lethal: Semtex
Tactical: Concussion Grenade/Portable Radar
Perk 1: Scavenger Pro
Perk 2: Hardline Pro/Assassin Pro
Perk 3: Marksman Pro
KillStreak Setup: Specialist - Sleight of Hand, Dead Silence, Steady Aim
This class is one of the most balanced in the game. The MP7 and MP9 have next to no recoil, and at medium range the suppressor can really help as the enemies will have no idea where you are. this class can be used very effectively for rushing, and the extended mags makes it easy to take out large groups of enemies. Hardline could be swapped for Assassin Pro as it will help conceal your position even more. Marksman on this class is very useful as it also doubles as a long range class - the extended mags (instead of rapid fire) on the MP7 helps the recoil, and so it is easy to kill people from a distance. Concussion grenades can be useful to make sure you win the gunfight before the enemy even knows you're there, and portable radar can be moved along wit you to make sure you know where the enemies are when you're rushing.
Also, as proof of how well this class works, check out the world's first Penta-MOAB - using this exact class: MW3: World's First Penta MOAB ( 5 in One Game) - YouTube
"KillWhore" Class Setup
Primary: ACR 6.8 Two attachements, Suppressor, Extended Mags
Secondary: MP9 Silenced
Lethal: Frag
Tactical: Portable Radar
Perk 1: Scavenger Pro
Perk 2: Hardline Pro
Perk 3: Stalker Pro
KillStreak Setup: Assault - Predator Missile, AH-6, Pavelow
You've probably noticed the similarity between this and my MOAB class. That's because it simply works so well! The reliability of this class makes it amazing, and without the super-soldier power of the Specialist Bonus on this class you're probably wondering how that is. Well, the killstreaks on this class are passive and will protect you while you run around. The predator makes it easy to get to your AH-6, and that will carry you to your Pavelow by protecting you from nearby enemies.
This class is also one of the best in the game. It was used recently to get the highest kill game ever on MW3: 186-4 w/ ACR Assault MOAB Domination on Resistance I World's Highest Kills - YouTube
Rush/Short-Range "KillWhore" Class Setup
Primary: P90 Two Attachments, Rapid Fire, Extended Mags
Secondary: MP9 Silenced
Lethal: Frag/Semtex
Tactical: Concussion Grenade
Perk 1: Extreme Conditioning Pro
Perk 2: Hardline Pro
Perk 3: Steady Aim Pro
KillStreak Setup: Assault - Predator Missile,
This works particularly well on Ground War, as it is an amazing spray and pray class; the extended mags and rapid fire make this the best primary for spraying down groups of enemies in close range, and the MP9 will help in any long range situations. I personally manage to get a quadfeed every match using this! The perk setup really help with the fast paced style of gameplay. extreme conditioning will get you in their spawn VERY quickly, and also safely out of any danger situations. Hardline speeds up the killstreaks and Steady Aim Pro really helps with the accuracy of the sprays and makes less need for a reaction time as you can ready your gun faster after sprinting. Again, the killstreaks are selected so that they will protect you as you rush.
Search & Destroy Class Setup
Primary: ACR 6.8 Focus, Red Dot
Secondary: MP9 Silenced
Lethal: Frag
Tactical: Flash Grenade
Perk 1: Extreme Conditioning Pro
Perk 2: Quickdraw Pro
Perk 3: SitRep Pro
KillStreak Setup: Support - UAV, Ballistic Vests
Last but not least, we have the S&D class I use. I do not play S&D as much as respawn game types but this class is still very useful. The focus and red dot on the ACR make it the most effective gun to win gun fights with in the entire game, as you take practically do flinch recoil from enemy shots. The MP9 helps when you want to be stealthy, and I have used Flashes on this class as Quickdraw Speeds up the usage time.Extreme Conditioning will get you out of panic situations and into unexpected spots quickly. Frag is useful for longer range kills, and SitRep Pro is VITAL for hearing unexpected enemies through walls, and also seeing their equipment before it's too late. Quickdraw is there for speedy actions and aiming, vital in S&D as it can all be down to reaction time. Support means that if you die, it's ok as you will still get the reward, and ballistic vests gives you an extra 50% health so it really can save your life.
I hope you've enjoyed a tour through the classes I use, and I hope you can benefit from them. Thanks for reading!