Capture the Flag is the classic capture and defend gametype from Halo history. This gametype can vary quite a bit from Two Flag where each team has a flag, One Flag where teams take turns attacking and defending or One Flag Neutral where there is a single flag in the middle of the map and teams constantly fight for control of it.
The most important thing to consider when entering a CTF game is what kind of role you will take during the match. If you are playing with people you don't know then it would pay to have a few different loadouts so you can change depending on the roles other players take. Most of the time, people will divide these up into offense and defense but during a Two Flag match (the most popular form of CTF) it is important that all people are ready to play either role as the flow of the match changes. For this reason, it is more appropriate to call the roles Offense or Defense oriented.
CTF matches can take place on any sized map but they tend to happen more on larger maps than smaller maps. Even still, most maps are large enough that they warrant a precision weapon like the BR or DMR. Plasma Grenades are slightly favored because vehicles frequently make appearances. For the same reason, the Plasma Pistol is quite useful.
Because these maps are generally a little bigger and can involve vehicles, Jet Pack isn't really recommended except for certain situations where it could help in capturing an enemy vehicle. Active Camo can be quite useful for sniping and the Regenerative Field is useful on defense for holding a position. Promethean Vision is also pretty useful both for offense (seeing the defending players) and defense (predicting the angle of attack).
Mobility or a Vehicular armor mods will probably be the most useful since you usually want to cover a lot of distance on pretty short notice and these will help most in that.
