I think Halo has a better multiplayer (here comes arguments) because it relies more on personal skill than crutches such as perks, profficiencies, killstreaks etc. There's no setup, everyone has an equal chance with the same weapons at the start. It also is geared more towards teamwork and when multiplayer games first came out, the point was to use teamwork which CoD has pretty much eliminated though I commend IW for trying a bit with Kill Confirmed. Obviously this was more between Halo and CoD and not Battlefield as Battlefield (at least BF:BC2) was also very team-centered and skill-centered.
Apparently Halo 4 will have custom classes. I heard of this from friends, not sure if true though.