I usually find myself pretty close to the top of the leaderboard everytime I play COD, I would say that I am pretty decent, I have like one or two classes that I use (M4 and AK) plus a sniper class and that's all I have ever really used! Your score will also depend on your play-style, my playing style usually invlolves running from area to area and then holding certain areas before moving on, never usually staying in the same place for more than 1-2 mins tops. Watching videos on YouTube can also be a big help as it gives you something direct to follow!