Three of my personal favorite combinations in COD4:

Weapon: RPD (Light Machine Gun)
Perks: 1. RPD Grip Attachment (w/iron sights)
2. Stopping Power
3. Deep Impact

The RPD Light Machine Gun might have taken over as my favorite. It's extremely powerful, accurate at long distances, has good iron sights, a huge clip, and with the added grip attachment gives a very strong punch at long distances. Fire off a couple rounds per burst to get maximum effectiveness. I was aiming this beast at random dots on my radar and getting kills left and right with deep impact. It's strong enough to let me make a mistake or two, while still overpowering my opponents with accurate bursts at short, medium and long distances.

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Weapon: M16 (Assault Rifle w/Red Dot)
Perks: 1. Frags x 3
2. Douple Tap
3. Extreme Conditioning

I'll admit, this combination is tough at close distances personally, but if you're good at getting the full burst into your opponent, this one is very strong. I add extreme conditioning to keep a bit mobile since it's a bit more of a medium/long distance weapon.

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Weapon: M4 Carbine (Assault Rifle w/Red Dot)
Perks: 1. Frags x 3
2. Stopping Power / Juggernaut
3. Deep Impact

The M4 is the go-to gun if nothing else is working for me. It's the one I trust the most since it's so well rounded and good at all distances. This one was also my most used until I discovered the RPD could have as good or better accuracy with only 2 round bursts.

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Of course, there's tons of other weapons to choose from, but I keep going back to these three for some reason. Maybe they just fit to my playing style or our gametypes. I had a run with the M40A3 sniper rifle for a while, but eventually I gave up on it because it just wasn't as effective as the assault rifles or light machine guns for me.