For a match or two, anyone can be. For the rest of the time, probably one of the pros who get paid to play it all day. Certainly not me, that's for sure - but back when I used to play around COD4 and WaW I never lost a FFA match for weeks and weeks on end. I played with some of the best players in the world at the time, and I was regularly matched with players in the top 100 on the leaderboards. Some of my friends have been in the top ten at some point. My friend Cheeky Devil was always right at the top on MW3, which is great as he never ever cheats or plays unfairly, hes not a camping exploitative bar-steward. Wudulikeaspud was in the top 10 in WaW and was another fair talented player.
For some reason, none of my friends are anywhere near the top of the Blops 2 leaderboards, for the first time we are all thousands below the top. From what I can tell. the players I have played with higher up the leaderboards play relentlessly with no mercy, they will use any advantage, wether it is unfair or not, they will camp a whole match if need be. There are also a lot of people at the top who have a huge score per minute but have only played few matches and happened to do really really well. SPM or KD ratio are only indicators of how useful you may be, actual win/loss may be very different in some cases - many people with great K/D ratios have low win/loss and vise versa.