In Call of Duty 4 there are a few types of map, which require slightly different ways of playing them (both with the weapons you choose and playing approach).
Small/Indoor Maps (Vacant/Shipment/Wetwork)
The small maps can lead to frantic action so you should be looking at the sub machine guns and shotguns generally. They can pack a lot of firepower and the accuracy/range of the assault rifles are not needed. This is unless you choose to stick to limited areas that have some range.
Claymores and C4 can be very effective on these tight maps used in and around doorways and stairwells.
Standard Medium Size (Crash/Crossfire)
The majority of maps offer areas where all guns can be used, there are indoor parts as well as long distances.
Large outdoors (Overgrown/Creek)
Sniper and assault rifles are needed here as range and accuracy is whats required. As there is more room to explore/hide in, the UAV jammer can be useful. Extreme Conditioning can also help cover sprinting the long distance.