With COD Black OPS 2, the leveling process is much like that of the previous franchises Call of Duty games.  You level from getting kills, objectives, kill streaks, and challenges.  Obviously the best way to level will be to just play the game, but the better you are, and the more kills you get, basically boils down to how fast you will level.  You can confirm this by just playing with a few friends who are much better than you.  If you really want to know the best modes for leveling the most efficiently, here they are.

1. Kill Confirmed – This mode is great, fast paced, and you get more points for grabbing the dog tags and the denied kill dog tags.  This adds up fast and IMO is the best mode for leveling fast in black ops 2.
2. If you are good, and consistent, search and destroy can grant a lot of XP, but you HAVE to get kills or you are better off basically playing any game mode.
3. Ground War – I like ground war a lot, but it fits my play style and most games I get atleast 15 or 20 kills so the XP comes in pretty fast, and in turn helps me get challenges done faster.

All in all, to level fast, play what you are good at, and stop reading so much and just play the game man! Game after game after game after game!! Enjoy and have fun with the game and the levels will come, and if you want to beat your friends, then just stay up later than they do :P.  Goodluck my nerd bros.

                                                                                  PWN LIFE